This is taken from another forum...
If your car is only used occasionally and on the weekend, then overdrive may not matter to you.
4R70W would cost alot more though (expensive). Also it's heavier and soaks up more HP than C4.
Powerglide is the lightest and eats up less horsepower. Only has 2 speeds but the 2nd gear ratio is 1:1 just like a C4 in 3rd gear.
On the highway the RPM of a Powerglide is the same as a C4.
The disadvantage of the Powerglide on the street is that 1st gear is very tall and you need good low end torque to get it moving (maybe your 408 makes good torque down low?)
Powerglides were production transmissions and were used on the street however.
With 3.27 gears and 26" tires at 60mph would be about 2600 RPM
The advantage of the Powerglide on the track is that the tall first gear makes launches more manageable when leaving off the line with twin turbo boost or 500 hp nitrous like you intend.
Plus the powerglide is cheap to build strong. Also you only shift once!
So... C4 is good and doable, but I quess it takes more money?
Keeping Ford as a Ford is ok, but if you only have a tube frame it's ok to use chevy parts. Ain't it Ted?
Seppo from Järvenpää, Finland
www.hollowheads.net (just click the hole in the head to proceed)