Thinking about a Gear Vendors OD. Can Anybody Give Some Guidance?

Here is my install;
When I got the unit in about 2002, it was only available with a long-tail adapter,
1) so I had to install a long mainshaft into the box; no biggie for me.
2) So I took the opportunity to also install a Passon alloy box.
3) The long-tail adapter is a B/E clone, so the front shifter pad is right inside the crossmember, and the front shifter boss hit that crossmember. It's aluminum so I zipped it right off.
4) Then I hung the unit and jacked the trans into position. The unit hit the floor-pan before the cross-member could be fully seated. I marked the tunnel by jacking the unit hard into it. It only hit the tunnel in two places. I dropped the unit, and went for my BFH. I clearanced the tunnel with a few well-placed smacks.
5) then re-installed the unit. Good job, the unit now hit the tunnel in a full continuous half-ring so that was gonna be as good as it gets. But the crossmember was still about a quarter inch short.
6) many years prior, I had installed a Milodon Roadrace pan with TTI headers, and to make it work, I had installed the later spool-mount x-member (with a poly mount) and jacked the engine up about a half inch for steering clearance. I chalked the lack of clearance at the GV to the engine not being in the factory place. No biggie; I dug out my die grinder and slotted out the spool-mount holes and bingo, in it went.
7) together with the Schumacher engine torque-strap, the unit has never rattled.
8) so OK, after the driveshaft was shortened and installed, the pinion angle needed to be adjusted a tad to get rid of a mysterious vibration. I took the opportunity to move the rear axle back, to the limit of the studs on the front spring hangers, about 3/8 inch, to install 28" tires.
9) that just left mounting the shifter.
You gotta build a new shifter mount, and you only have the rear mount on the tail left. I could have built that mount to reuse the factory floor-hole location. But I never liked that location in the first place, And, having bucket seats, I thought to myself, that here is a golden opportunity to put the shifter where it shoulddabin in the first place. So I moved the stick back about 7 inches. This opened up the option to also move it laterally, so I did. This opened up the option to raise it. Since I had this ancient Mr.Gasket Bang! short-stick Shifter, I raised the thing high enough to install the top bolt from inside the cabin. So the shifter ended up falling to hand with me in the saddle, and my arm hanging near vertical. So again, I used this opportunity to adjust the height of my bucket, and that was that.
10) Ok that left the shift-rods to deal with. I went to the machine-shop and got me some tubing with the right-sized center-bore for fitting the cut-off parts of some old Chevy shifter rods that I had kicking around. I think the tubing was called "seamless" and maybe "cold-rolled", and it did not have the usual center weld-seam in it. Anyways, I bent me up some nice solid rods, and have not, since that day, ever missed a shift again. I think, IIRC the center hole of that tubing was 3/8; but it mightabin 7/16. In either case, they are quite "robust".
11) finally, I had the Chevy speed-o adapter to deal with, but the GV folks supplied the right parts, the first time, for the 3.55s that I was running. Of course every time I changed chunks, this required a trip to the Chevy place for a new gear. I now have a box full of them, lol.

So OK, it's NOT a straight bolt-in;
but the fact is, that clearancing the tunnel only took a few well-placed hammer-blows, to stretch the metal in a couple of places. This was the smallest problem of all for me, and in my case, was mostly due to the non-stock location of the engine.
My car is also lowered, and on softer street springs, and on a narrowed rear end, with offset springs; so I may not have set the pinion angle quite right in the first place.
So I mean, some of my challenges might have been self-created. But I want to re-iterate;
clearancing the tunnel only took a few well-placed hammer-blows.



As for "only" a .78 overdrive ratio....
Well, whatever;
it drops OP's 3050rpm with 3.55s, to 2380, which is 670 rpm, or 22% of 3050, which is the equivalent of dropping those 3.55s to 2.77s; so ................ I fail to see .78 as a problem.
In fact, if it was any lower, and the engine has a bit of a cam as they say, it would be as good as impossible to give the engine the cruise-timing it needs for optimum fuel-economy. ....... which is more than half the reason I installed it in the first place. I saved the cost of it, in fuel-savings, in less than five years of use as a DD, and, in the meantime, enjoyed the other benefits. If I stopped to do the math, I probable saved it in three years.
Oh, and, my 367 now has well over 100,000 miles on it, and a recent compression test shows that the pressure is still up there. So I mean, the GVod fits into the OP's original post quite well. Like 2380rpm is a very nice cruise-rpm. and I showed how to optimize even that.

Having said that;
OP
rev up your engine, in Neutral, until it the vacuum levels off. Whatever the rpm gets to, you can consider that the lowest efficient cruise rpm of the combo, in it's current state. My guess is that it will peak in the range of 1800 to 2400, depending on your cam.
Say it peaks at 2200.
Now, without regard to the numbers, keep the rpm at 2200 and just keep advancing the timing until more advance does NOT produce more rpm. Now read the timing at that 2200rpm. Write it down, then put the timing back.
Now, my guess is that the number will be in the range of 50* to maybe 56*. Whatever you read is what the engine wants. How are you gonna give it to her?
Your Vcan is worth what? Maybe 12 degrees?
Your idle-timing is what? Maybe 18*?
Your centrifical, by 2200, is bringing in what? Maybe 10*?
for a total of 40*?
How are you gonna stretch that to 50/56? If you don't, your cruise-timing will be seriously retarded; and your fuel-economy will be seriously lacking.
You can modify your Vcan to in the range of 22/24, so from 12, let's say you get an extra 10*, so now you are at 50* and on the lower edge of adequate, but could be still 6 degrees or more, short of optimum.
Can you get it by increasing idle-timing to 24*? Or by increasing the rate of the centrifical? Well maybe; but if this causes detonation under Power, then no.
But increasing your Cruise speed to 2400, might get you 2 or 3 degrees of centrifical, and now yur up to say 53 degrees. Perhaps that is all your engine wants.
Go back and repeat the cruise-timing test at 2400 and see.
Happy HotRodding