1st gen dodge Transmission swap or gear splitter?
 

My 1991 D350 has a 3 speed auto transmission which is not friendly on the highway, I'd love to get the revs down. Should I install a gear splitter, or can I swap out the 3-speed for a newer better (Allison) transmission. Discussion is appreciated got the $$ set aside for whatever is best.

i suggest Gear Vendors https://www.gearvendors.com/hrdodge3s.html

With a GV you still have converter slip.
Depends on what you use the truck for, a later trans with Lock up and Overdrive maybe a better option

I'll be towing a 26 foot boat, about 9800 lbs

the awesome thing about Gear Vendors is their warranty, you can always get a better converter, i think going with a newer auto trans is a bit more difficult as you have to get a Throttle positioning sensor, and a wiring harness for it, or you could go to a 5 or 6 speed manual trans like i plan to do. with either GV or different trans you will have to shorten the drive shaft and have it re-balanced.

I would look for a cheap 1stgen D250/350 5speed truck, and start swapping parts. This way you get everything you need (clutch business, trans, driveshaft, appropriate axle gearing) plus a multitude of spare everythings for your truck.

yea ive been thinking of doing a g56 swap been reading a few forums where people have done it, and its not too hard of a swap, all you need is a 2nd gen adapter plate and a 2nd gen starter and trans mount, then get the clutch of your choice, along with a flywheel, most likely will have to shorten the drive line

Depending on production year, the G56 had two different ratios for 6th. When I went from auto to manual, my overdrive went from 0.72:1 to 0.8:1. May not sound like much, but, I traded off 10% of my top end, and about 10% of my highway MPG, in order to get more reliability. You going from a 1st-gen 3-speed non-locking trans will probably gain some MPG. Do it if you have the chance.

i know ill gain some MPG im more interested in top speed, my top speed now with out a 366 spring is 80, id like to go a lil faster to keep up with some dirty max's around here

If you're looking for simple, I would go with a stout rebuilt non lockup A518 off the IC first gens and be done with it.

No lockup tranny starter/adapter plate/driveshaft modifications, one wire/switch to control overdrive, and good converters are cheaper than the lockup units.