Back in the day the two big modifications to the TH400 were a "Bullet proof" sprag and a Cadillac clutch pack due to its increased clutch capacity. Now of course that has changed, our cars have more horsepower today.
As for fluid, B&M always recommended Type F fluid (Before they marketed their own) because of the Ford friction modifiers. I have used Type F since the seventies in all my automatics(3 speeds) and have never had a problem. I have never used a synthetic in any of my traditional automatics as of today so I can't speak about that. I do question the reaction of synthetics on older clutch lining material since that's about the only fluid available today as well as being more leak prone. Trans fluid is always supposed be backwards compatible but I don't know if they considered technology that old. JMHO
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1978 Black & Gold T/A [complete 70 Ram Air III (carb to pan) PQ and 12 bolt], fully loaded, deluxe, WS6, T-Top car - 1972 Formula 455HO Ram Air numbers matching Julep Green - 1971 T/A 455, 320 CFM Eheads, RP cam, Doug's headers, Fuel injection, TKX 5 Spd. 12 Bolt 3.73, 4 wheel disc. All A/C cars
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