I don`t know how fancy your C6 was, or what 4 speed you have, but when I converted my 428 CJ powered 78 Fairmont, from a C6 to the Jerico 4 speed, the improvement was considerable. My C6 was pretty decent, needle bearing low drag internals, "R" servo, Art Carr reverse manual transbrake valve body, and 8 " convertor (4500 RPM on the foot brake, 5200 at WOT with the transbrake), with the C6, the best numbers were 10.55ET at 126 MPH, with a 1.42 60 foot. When I first installed the Jerico 4 speed, I changed nothing else, other than swapping the 4.56 gears for 4.88s, since there would no longer be any convertor slippage. On the very first pass with the 4 speed, leaving at low RPM (4500 RPM), and hitting the rev limiter on every shift, it still went almost 2 tenths quicker and 2 MPH faster than the best run with the automatic. Getting used to the car with the stick resulted in better numbers, eventually ran a best of 10.03 @ 132 MPH with the identical 428, with a best 60 foot of 1.28. And that was after putting the 4.56 gears back in. (There was no convertor slippage, but the added 6 MPH put the finish line RPM up to 6800 RPM, beyond my comfort zone for my 40 year old stock blok crank, and rods, and higher than my flat tappet really wanted anyhow).
