Depending on the depth of your wallet, there are lots of internal upgrades you can do. However, I think that with you looking at the 400HP range I wouldn't get to excited about some of those items. If it was mine I would go with a total factory rebuild: friction plates (use the high energy frictions in the Hi/Rev, forward, and overdrive), complete steel kit, gasket and seal overhaul kit, both bands, bushing kit, both sprags ,and a filter. I'd also check the bearings in the OD section. Change the OD planetary if it has the old early design 22.5 degree tooth pitch. If so, change it to the later 15 degree pitch. The early ones were troublesome. Typically there is a kit available with all the parts required to make the change over. When taking the OD section apart, be awful careful. there is a big spring in that section that is under compression. I've heard about 800 pounds pressure. I'd check all the electronics and make sure that they are OK. For a shift kit the TransGo TFOD-HD2 is an excellent kit. Converter wise, call Paul at T.C.S. out in Langley B.C. 1-800-960-1177, give him all your engine/diff/tire information and he can come up with a converter best to suit you requirements. I believe that you can get a lockup style converter in the 4000+ stall range.
Have fun and do it right "ONCE"
