rebuilding a 273 180 HP engine

I agree with checking the engine first. If it is a good "air pump", no need for a rebuild. One test is compression. Should be >100 psi in all cylinders. Hard to say more since depends a lot on the cam, but all cylinders should be +/-10 psi. Another test is to turn the engine over by hand from the crank bolt. If you feel each cylinder strongly resist you like a spring, and each holds pressure >10 sec, the "air pump" is fine.

If a good air pump, but low compression, might have a wild cam with a lot of overlap (bad idle, but tuners love the thumpy sound).

If a leaky air pump, apply air pressure to each spark plug hole with the valves closed (piston at bottom) and try to see if it is hissing past the rings (hear in crank-case), intake (hear in carb inlet), or exhaust (hear in tail-pipe).

Re some fixes. I have tried the "hold valves up w/ air pressure" trick and don't like it. You must jam the engine in position or it may spin on you (has on me). I just bring the piston to TDC (use a wire in spark plug hole to verify), then the valves can't drop into the cylinder. You can pull the valve up w/ a magnet to hold while removing the keepers. Others suggest stuffing rope in the cylinder, but extra work. If you replace valve springs, double-check the closed length. I could just barely get some keepers in, especially on valves I shimmed tight to make spec length.

I switched to a hydraulic cam (common .422/.440 "RV/Torque"). The shorter pushrods needed were only ~$35 at Summit. I used Roades "leak-down" lifters as an attempt for better idle and mileage. Haven't driven yet. Keep your adjustable rockers, they are best and work fine w/ hydraulic lifters.