400's were 6 bolt
the 203 trans has the shortest output shaft stickout. 208/231/late model 205 are the next longest with fig 8 205 rounding them out. I'm not sure your 203 trans will reach his long input (will prob reach, but won't have ample spline engagement. )
You have the 1.5" stickout trans and he has the adapter and input for the 2.375~ (from memory) stickout for the middle length shaft. Check first. Either way, if you want a 205, you need one fo the 2 longer outputs. the 400 out of a cucv works with Jimmy's setup. Your 203 one works for guys wanting 203 doublers, and atlas folks, as the atlas and adapter come setup for the 1.5" shaft. This means you have some bargaining chips unless your trans is built and you don't want to part with it. If not, you can probably swap or sell for a cucv or 81-90 4x4 trans