I'm building a a500/42rh for my 66 Barracuda. You'll know if you have a 42rh if the female connector by the shift selector is a 3 pin. If it's a 42re it will be a big round connector with about 8 pins.
The 42rh can handle 300hp very well. If I were you, I'd put a rebuild kit in it with alto red clutches and kolene steels, upgrade the rear/reverse servo to a super servo (from Superior Transmission), and a shift kit at the very least. Here's what I'm upgrading on my 42rh to get it ready for 450hp/510 torque:
Fairbanks level 2 shift kit
Bolt in sprag (over running clutch)
New Bushings
Reverse super servo (prevents piston flip/piston breakage)
Upgraded intermediate servo
Billet accumulator piston
Wider front/intermediate band, I had a 1.375" and I'm putting in a 1.60"
Kevlar low/reverse band
Overdrive planetary upgrade from 4 to 5 pinions, change gear angle from 22 to 15 degree
OD Direct clutch - upgrade from 6 to 8 clutch discs
OD Clutch - upgrade from 3 to 4 clutch discs
Intermediate clutch - most came with 3 or 4 clutches and need to be upgraded to 5, mine came with 5
New oil pump hears with newer 'flat' drive instead of slotted drive
Upgraded 500hp Torque converter with flat oil pump drive, not slotted pump drive
There are probably some valve body modifications to be done, I haven't finished my research on that yet.
There's also a TSB out on the OD piston to replace it with p/n 52118599 ($29 online) which I'm going to do.
That, a manual, and a few special tools (dial calipers, micrometers, torque wrench (foot and inch pound), trans assembly goo, OD spring compressor, Intermediate spring compressor, tools 6311 (5.50" spacer) and 6312 (.50" measuring bar) all shop made for next to nothing), and your on your way.
I'll probably have about $1000 in mine counting the $100 core and the torque converter, but not counting my time. But you really don't have to do all of this... for 300hp, I would probably do the alto red clutches with kolene steels, shift kit (your choice, many like the Trans-Go, I went with the Fairbanks), and low/reverse Super Servo.
It's really not expensive to upgrade the clutch counts, you just have to be aware of the space tolerances and order clutch plates, pressure plates, and selective spacers accordingly.
There are a lot of videos out online, you'll even find the 727 rebuild videos very helpful. But whatever you do, get a printed manual, read read read, and take your time. If ever there were a project that begs to be done more than once, it's a transmission rebuild.
