whats a good alternative transmission to replace the 4l60e out of the pre 99 silverado. or what would need to be done to the 4l60e to handle 400-500hp, and where could i find those parts. thanks

engine plans are either ls1 or carbd 350/383

 

th400 with a gear vendor

 

You cannot use a pre-99 4L60-E because those would have Gen-I/II bellhousings and input shafts. It can be hard to tell by looking at them, 2-piece cases started in 97 on small trucks and 98 in full size. Make sure it has a mounting hole in the top center of the bell.

Also, I agree, a TH400 with a Gear Vendors Overdrive would be nice. Or a 4L80-E. Anything but a 4L60-E. At 4-500 HP you can only expect about 25K miles out of a fully-built 60.