there a thread on here that shows the differece between the at545 and mt643. those are the 2 most commonly found transmissions on the older ones. if its a 5.9 i'd almost expect the AT545, if its and 8.3L its probably the mt643.
the at 545 gets a bad rap. its a work horse. we are just close to the hp limit of it.the more you can do to take care of it, the better it treats you. i added an oil cooler to mine and the result was like adding a turbo charger.... it just ran better.
modding a bus and modding the ram truck are 2 different bears. imo, turning the screw up much with the at545 transission would kill it.
i pull a trailer with my bus, and it does it just fine. I'm in colorado so my trips are pretty much just hill climbs.
the governor springs swap is the most cost effective way to get more top speed. but when i'm in the bus, towing, i'm 24K lbs. and getting that mess going much faster than the 70ish it can will get you into trouble quick. for the 5 years of ownership, I got used to going a bit slower. i'm not the slowest on the road, i can still pass a jeep if its towing something.
if you want more power, you should started with it. get an 8.3cummins with the mt643. thats cheaper than beefing up a 5.9. most mechanics will decline modding a diesel, just for the shortened longevity of the engine. these are things we got to do at home. so its an at your own risk, from a shop.
a year ago i played with the idea of swapping up to an mt 643. but its not a straight swap. its a different bolt pattern, its a bit longer, but if you have the ability to do it, its possible. the thing that scared me away from the swap was top speed. it wasn't going to change with a trans swap. swapping a 4 speed for a 4 speed with the same final drive ratio doesn't gain any speed. the cost/benefit isn't there for me.
maybe the 3k springs wouldn't be to hard on it. i'd like to see you do it first!
there are not to many threads of diesel bus mods. one by a guy named lapeer - i think he slid the fuel plate and was happy. that's the only thread i know of. i was really surprised by the improvement from my oil cooler.
i have a high mileage cummins, 300K+, and it got that way just the way it is.