Can you post the casting numbers? If your heads were fitted with 2.02/1.6 valves; I highly doubt your friend built you a 283. Also, a 283 usually needs 5-600cfm; so a 780 would probably behave badly.
Thanks for your service, post any casting numbers or part numbers used inside the motor for the rebuild.
Even a 327 would struggle to make '500hp' with conventional off-the-shelf parts
1970s 331ci (overbored 327) Pro Stock engines struggled to make 600hp and thats with a few days in porting, cutting, welding heads and 13:1 compression or more.
Theres no reason you cant put a T56 behind a 283; what are your goals for the car.