Edelbrock / RHS heads, or go BIG block and put it to bed!
heads were done at the motor builder. im sure not a real pro job. done with a hand polisher. no flow test. whats a leak down test? I bet if i put edelbrock heads on the motor would bring it back up to where it should be?
A leakdown test is similar to a compression test, but it tells you how much pressure is being lost past rings or valves. It's more of a performance compression test...lol. The reason I bring that up is the math is telling me what you have now is not producing what it should. IMO, adding any new parts without addressing that issue, might take care of it (if you replace the right stuff) or might do nothing (if you don't).
You have a car that weighs (estimated by me) around 3200lbs. If it's actually heavier, you're making a little more power. If it's lighter, it's making less. You reach 100mph. With my estimate that calculates about 250hp to the tires, or around 300 at the crank. Your dyno confirmed it's close to that. There's always some error in figures such as these so don't start looking for the difference between them...lol. Just understand that it's low for what you have for parts. What I think might be the case, is the lower end is not blueprinted and the static compression is lower, and/or the heads have problems, and/or the cam or installtion has problems, and/or the tuning is off. But, I would anticipate if the engine had ported heads (assuming mild and average quality) and a 280/.480 hydraulic cam, with 9.5:1 and the parts you mention, that you would have closer to 375-390hp and be running closer to 107 in the 1/4. Which is high 12s and what I would expect to see a 360 A body running.
Now, if you are dead set on heads and the leakdown shows no ring seal problems: IMO, you should pull a cylinder head first looking for what you have for pistons, piston installed height (how far down in the bore they are), and valve reliefs. If the compression is really 9.5:1 now, then adding the closed chamber heads will add about .8 to the compression ratio. Depending on the pistons and how it was put together you may need to be careful with the cam choice. If you go with aluminums, you have to use the right gasket and it's thicker than the MP thin ones that would work with iron. So again, to get everything from it, you need to get what you already have on paper nad then come up with a plan. $2500 will get the job done with RPMs too so budget's not really an issue at this point.