Gear Vendors or swap for performance 5 spd OD?

My 2 cents is to get a chevy 4 speed auto

While gear-splitting sounds exciting, it doesn't work particularly well with a stock-cam 318/904, on the street.
The 904 ratios are 2.45-1.45-1.00
The GVod is .78. Combining them you get
2.45-1.91-1.45-1.13-1.00-.78od.. The splits are
.78-.76-.78-.88-.78

With the od, you are likely gonna shoot for a 2.76 final drive so 2.76/.78=3.55s in the back.

Minor performance upgrades, usually means still running the stock cam. Therefore, the power will peak at about 4000rpm, and your shift rpm could be 4400.
Ok lets convert this to mph at WOT. Assuming 27" tires;4400 is;
38 in first,
48 in first-over,
60 will be 4140@ 8% slip, perfect
64 in second,
82 in second-over,
93 in direct.
93 is gonna take you ~16/17seconds, so you better choose your spot carefully
64 will be your everyday cut-off, so you are paying $2850 to split one gear, and for the overdrive cruiser gear.
The thing is this; The stock cam 318 loses torque so rapidly after 3000, that the horsepower curve is sorta flat from there to the peak. So splitting that gear may possible gain you something, in the zero to 60mph contest, but my money is on about nothing
The other problem is that the GV requires a power-cut to outshift, which you would have to do at 48 mph to get into Second. So very possibly, probably, any time you gained on the in-shift, you are gonna lose it all, on the outshift.
So, honestly, the GVod is just gonna end up as a novelty, and a cruiser gear. $2850 is a lotta coin for just a cruiser gear.
IMO
that money would be better spent on a 5.9 Magnum and leave whatever gears are in your car, but install a 2800TC.

Maybe you already have 2.76s
The 5.9 at 2800rpm, will put down about the same or probably a lil more power on the launch, than your LA318 with 3.55s, and it will hit 60 at ~5300 at the top of first gear,probably dwarfing the 318's ET. 5300 is probably a lil high, but just put some appropriate valve springs on her and letRbuck.
On the shift into Second at 60, the 5.9 is all set to run to 93mph @5500
And of course , it will cruise at the same Rpm, as the 904/GVod.
So this is IMO, a better solution, if I'm reading you right.

Now, why did I recommend the 4L60 /700R4 GM trans?
Cuz the ratios are
3.06-1.63-1.00-.696od, and the splits are
.53-.61-.70 nice progressive splits for the 318 .
Again for a final drive of 2.76, this would require 2.79/.696=3.91s, booyah

Now lets compare this at the rear wheels;
first the 904/3.55/GVod; then the 4L602/3.91s.
1).. 8.70-5.15-3.55-2.77
2) 11.96-6.37-3.91-2.72......... Ok but your still thinking splitting so;904/3.55
1B).. 8.70-6.79-5.15-4.02-3.55-2.77, but hang on; this GM is gonna hit 60 @5130 @ 8% slip, That is too much for your stock-cam 318. In third it will be 3150*, too low. So we need different gears to be fair. The closest gears are 5.15/1.63=3.16 so round to 3.23s and now the numbers look like
2B) 9.88-5.26-3.23-2.25 whoaaa. Compare this to the 904/3.55s
1) . 8.70-5.15-3.55-2.77
Look at that! Both first and second are still better. We lost 10% in third, but man-oh-man, 65=1817 rpm, real nice for the factory 318 cam; you should be able to get about 7 million mpgs,lol.
What do ya think about that!
And even better,the GM has a loc-up TC.

And finally; yes you can get adapters to put that Chebby lump into your Mopar.
The Mopar has a deeper od (.69). and the ratios of the A500 are tighter. Which in theory would be a better choice for your 318, but making it fit is a PITA.
The ratios are 2.74-1.54-1.00-.69
for a cruiser of 2.76 this would require 2.76/.69=4.00 rear so lets round to 4.10s.
On the road then this would be;
4) 11.23-6.31-4.10-2.83.. compared to the GM
2) 11.96-6.37-3.91-2.72.. see the edge in third gear there? just where the 318 needs it. The penalty is 80 more cruising rpm with the 4.10s. And the A500 also has a loc-up. I did not correct these to a 5.15 second gear, cuz I don't see a point in gears that low for cruising, or that high to blast off with. A better idea would be to up-came the 318.

My money is on dropping a junkyard 5.9 in there with a4bbl and a 2800TC. And leaving the rest alone........... 4now,lol.

done