I know of a few guys who just slapped in used 700R4's from a gas powered truck into their 6.2 trucks and left the torque converter in them ,they noted only a slight increase in the stall speed and one guy decided to swap the govenor weights from his old diesel tranny into the swapped one,to lower the shift points a bit...
The diesel converter has a bit lower stall speed,and 6 bolts to attach it to the flex plate,gas engines only have 3 bolts...the 3 out of six bolt up to the flex plate though,and didn't seem to be an issue for those guys,who mostly just drive on the road and use their trucks for light duty chores..
I have seen some 80's vans and pickups with 292 sixes and V8's that also had the 6 bolt "diesel" low stall converters factory,so I dont know how big a deal it is to use one behind a diesel..I prefer the lower stall myself,it feels more "direct" with less slip and rpm buildup before taking off...
A TH400 converter will fit on a TH350,if you wanted to use a TH400 diesel style one,you probably wont find one for a TH350 too easily...not sure if GM even used TH350's behind diesels,if they did,it was probably only for a few years in the early 80's..Edit:--the TH400 converter isn't compatable with a TH350 that has a lockup converter,as far as I know...
You'll need to either hook up the vacuum pump on the engine to the TH350's modulator with a VRV valve (vacuum regulator valve),in order to get it to shift properly...the VRV switches are costly and getting harder to find,unless you score one at a junkyard,most places balk at parting out an injector pump though,and wont sell it separately...you can also re-work the govenor to allow it to upshift with little or no vacuum to the modulator...
Greg Ducato helped me out with this problem when my vacuum pump crapped out and my Th400 wouldn't upshift right,it wouldn't go into third until 45+ mph...he sent me a modified govenor and a spring to put on the modulator valve,now even with no vacuum to the modulator,it shifts into 3rd at 35 mph or so,which is much more bearable than having it hang in 2nd around town till you hit 45+...I have since got another vacuum pump from a member here,but haven't gotten it installed yet--I dont know if I'll even need it hooked to the modulator now or not..
I never took a transfer case apart myself,but I watched my brother take the input shaft out of a NP-203 years ago,you have to pull the case off the tranny and take it out from the "front" side...there are a bunch of needle bearings in there that'll drop into the case I believe,so its best to remove it very carefully with the case and input sticking straight up..if you lose any needles,you'll be fishing them out of the case thru the PTO cover,and don't miss any,even one left behind will cause a ton of grief and damage the gears or bearings..
