340 low compression

I bought a 340 a few years ago that was machined by a reputable machine shop. It was a 73 with new cast flat-top pistons 8.7:1, j heads with small valves, cast crank, comp extreme energy.477-.480 cam and I have been thinking about shaving the heads for more compression. Car is basically a cruiser but dynoed at 275 h.p.. My Mazda wagon dynos at 275
Any tips?
I have headers, fuel injection, and older performer intake. She does run good in a 67 belvedere. 3.23s, r.v. Torque converter

Maybe a Pro charger with 2 to 6 lbs. boost is the easy way Feels good, last a long time

Either Enginequest or Eddie heads. You need something on there with smaller chambers and flows better.

I live at 5,000 feet and think compression is much needed. Being on a budget, if I can mill my heads .40 and probably have to buy shorter pushrods what else would I need?
Any other budget friendly ideas?

maybe some OE mag heads if you found a set in good shape/not needing alot of $$$. but there is is the intake/rockers etc for that swap. Is the 8.7 actual measured?

Yes-8.7 measured

I do think the combo could benefit from more compression... and still run pump fuel. Rule of thumb, low 9's CR is OK with open chamber iron heads at low altitude. If you stay at higher altitude, higher compression should be OK (unless the gas is lower octane which may be the case).

Gains would not be huge. Is it worth it? Up to you.

What about a Hughes whiplash cam? They have been out for awhile. Will it help with the lower compression much more than my extreme energy cam?

How much can I take off j heads and be safe?

Choosing a cam to adjust the compression ratio is ineffective.
To get an idea of the effect, advance yours (if you have the intake P-V clearance).

That is one deal where the only answer is more compression--IMO I would drive that good running combo forever and keep that $$ in your jeans but--if you want it to pep up do it right and swap the pistons leave the rest as is
Milling the heads just can't make up for a piston that is so far down in the hole that is like water in a well
Just drive it or.... put a 125 shot of NOS on it and for $400 and two hours you have done the most with the least and made it a real Hot Rod that drives easy otherwise
Perfect candidate for a 125 shot I mean Perfect!