the difference is TQ splines has to do with the year of the transmission rather than what it came behind. you cant use a 1970 TQ in a 1969 transmission.Well I learned the hard way there are 2 different sized torque converter splines. I was not on this forum when I put my 289 in & was never a Ford guy. I asked a bunch of different people & done a lot of googling every one said the six trans will be fine just use the V8 converter. I had a friend over helping me & he put the converter in & it slid right in. It was all good until we started it & put it in gear. :bitch: That is when I read more and learned about the different sized splines. GRRR Well it ended up with a V8 trans in it a couple days later.
as for the rest, the clutch packs have one less clutch in them, the servos tend to be a bit smaller, etc. you can add back the one clutch in each clutch pack though, and you can change to the larger servo pistons as well in some instances.
but even if you just rebuild what you have and upgrade the better components, ie better clutch packs, the six cylinder C4 will handle a mild 302 in normal day to day driving, and even the occasional ragging at the strip(that means a couple times a year, not every other week).