71 340 Compression Test

I will get with my engine guy on the cam selection and fast lobes. I gotta be honest in that I like a little lope to an engine but still street able.
You guys might be able to shed some light on the actual hp of a 1973 340? The data that I have seen shows to be about 230 hp with the small valve heads? I have heard that the x-head 68-69 340's were advertised at 275 but we're more like 300-325? Is it anyone's opinion that the 73 may be under rated as well?
This car has a A833 4spd (not sure if fine or course spline) I do know that it has a 489 rear end. These has the heavy duty axles in them, right? Also has SureGrip but no idea of gear ratio.
Sounds like I need to get block re-done, decide comp ratio of Pistons, have heads re-worked with larger valves then let engine guy check the CR and valve clearance with heads on to make sure. While he is working on engine I can verify gearing?
Any idea how many ponies those factory manifolds are good for? At least initially for the car shows it might be nice to look original on the outside.
Appreciate the good information guys.