71 198 slant 3 on tree

Is this project for fuel-economy purposes? What are you hoping to gain, and what is the projected pay-back period?
I did a similar thing.
By gearing your engine down with overdrive, the cruise rpm would get so low, that IMO a Single-bbl cannot atomize properly. The smaller venturies of the 2bbl solved that for me, and the fuel economy on the hi-way went up.
However;
1) if you are switching to port-EFI, this is of no benefit because the carb becomes just an air valve, so you can install a "carb" with any size butterflies or venturies. However, for driveability, you need to use good judgement, because with a 1:1 ratio, she gets jumpy with a pair of too big valves, and after a point, the fuel curve would go very lean.
2) if installing TBI-injection on the SS 2bbl with the injectors above the valves, well IDK; I suppose it would depend on the size of the injector(s) cuz I suspect there would be very little room for air to get around them. Plus, TBI on this application, (stock 198slanty/ non-overdrive), for fuel-economy,
IMO, to put it kindly; Is a waste of time and resources.
But, with the injectors below the valves, IDK either, but it would be better than above, and probably easier. But again;
even with 2.76 gears and 65=2500rpm, that EFI will be very little better than a well-tuned carb........
Until you get the cruise-rpm down to sub-2000, where it becomes impossible for the stock distributor to supply the correct amount of cruise-timing, EFI will be of very little benefit. Whether or not your 198 can generate enough power to cruise down there, I cannot say. You cannot take the rpm down so low, that it requires WOT to maintain your desired cruise-rpm. Well I mean you could, but...

IMO, putting an overdrive in it, together with re-engineering the ignition timing curve, will reap far more benefits; but if you get below about 65=2200, you will need some kind of a stand-alone timing computer.
Why?
Because by that rpm, the factory distributor cannot be re-engineered to supply the correct amount of Cruise-timing AND Power-timing AND Part Throttle timing, all with the same, even modified, factory guts. And without the correct Cruise Timing, fuel-economy will just get worse (or at least not get any better) because the throttle will have to be way too far open to get the required cruise-power. So what you gained in rpm-reduction, you lose in the inefficiency of the loss of Cruise-timing.

To see how this works;
Rev your engine up to 2000 rpm and fix it there. Now just grab the distributor and start advancing the timing without regard to the actual numbers.
As the rpm goes up, reduce it back to 2000.
When the rpm no longer rises with more timing, now your timing is optimized for that rpm. NOW read the balancer marks. Don't be surprised to see a number well over 35degrees, and possibly closer to 40/50 degrees. My 360SBM, for example, at one time, liked 60 degrees@2000 .. Of course, under load this will change slightly, but may change back again as you lean out the cruise circuit.
The key to fuel economy lies in rpm reduction and adequate ignition advance, THEN AFR optimization. To start with the AFR, you might as well stick with a carb.
The above test will get your Cruise-timing ball-parked. But you cannot drive it when it is set to 54*@2000! else you may rattle all your piston skirts straight into the oilpan.....
To get numbers as high as 54 degrees, you have to modify the V-can which maxes out at between 20 to 24 degrees; leaving ~34 in the mechanical. Which can be done... but 34*@2000 will detonate at full-power, and possibly even down to as low as quarter-throttle. Detonation must be avoided at ALL COST; cuz it breaks stuff.
This is the reason for the stand-alone timing computer.
Any way, that is my spiel, hope you get something out of it.
Happy HotRodding.

EDIT
BTW; with a manual trans, you are already set up for an F-body A833od. It has about the same ratios as your current trans, but has a .73overdrive gear. From 65=2500, that od gear would drop your Rs to .73 x 2500=1825 by the math.
The only hiccups are your shifter, and probably/possibly, the BH and driveshaft slip-yoke. That .73overdrive, by percentage is said to increase your fuel-economy by one-half of the rpm reduction, so that comes to;
1 less .73 times .5= 13.5%. If you are already getting say 20mpgs, then the od might be worth 2.7mpgs, after you get the timing worked out. Your TBI-EFI by itself with no other changes is not likely to touch that.
I buy my A833-ods at swap-meets with beer-money, cuz guys often don't want to drag them home anymore. I scored a minty one once for $50; the guts were like new.
You can modify your column-shifter for 4 gears but would have to figure out something else for reverse. It's already been done by at least one FABO member.
If you have a B&T front yoke, that just leaves changing that out.
If you were so inclined, you could increase your rear gear, to end up at 65=2000 to 2200, and gain some off-the-line performance. Say 2200, that makes it possible to modify your current distributor closer to optimum, and could be fully optimized with just a dial-back(DB) timing module used strictly for cruising. Mine has a working range of 15 degrees. I can change my timing from the front seat to anywhere in that range.
Say you modify your V-can to 20*, and set the DB to 10*, and say your engine likes 54* at a cruise of 65=2200. You got up to 20+10 =30 degrees already, so in the mechanical, you just need 54 less 30= 24@2200 and that might be just fine at Full-Power. If not, you still have 5* of retard in the DB to play with. I love that tool!
The only hiccup with this tool is to remember to Zero it when you come off the hi-way. Mine, a Jacob's, is now no longer being manufactured, (at least I cannot find it on the Net) ...... but MSD has one.