when you had the battery and starter checked, did they do a load test on the battery, load at 50% of the cca rating for 15 sec, if volts go below 9.6 then the batt is toast.
I would do a check on the timing setting, or just try retarding the distributor some and see if it fires. the timing for initial start up is a guess anyway, and should be final set on an engine at operating temp with a timing light, after the carb etc have all been adjusted.
if your heads were done at a reputable shop, the valve height should be ok. if it was overlooked the valve would be protruding too far out of the head and would cause the valve to be open if anything, which would mean less compression. grinding the valve seats causes the valve to sit deeper into the head, so stick out the back side more, so cause tight valve clearance. a new valve job shouldn't give you any more compression than what that engine had when it was brand new. that starter worked then, and has been checked as ok, so what else changed.
check this stuff:
- battery connections. not just the terminals, check the wire as well. sometimes they are green down inside the wire. usually can tell that because the wire is quite stiff next to the terminal and then down a way it gets more flexible. it will pass enough power to crank the engine over without the plugs but maybe not enough for when the plugs are in.
-cable connections at the starter solenoid and at the starter. you can do a voltage drop test to make sure. if it is a stock style starter with no solenoid attached, just a cable from the fender mounted solenoid, take the cover off the side of the starter and make sure that the contact under there is in good shape. when those starters get power, first the bendix is slung out into the flywheel teeth and then that set of contacts connects and sends battery power through to the starter motor, so the stater doesn't grind it's way into the flywheel teeth every time. I have had engines do what yours is doing and that was the problem.
-cable connections at the ground connection to the engine block. not on a painted surface, clean and tight. wouldn't hurt to take it off, clean with sandpaper or something to get down to shiny bare metal, put a star washer under the cable end just to make sure all is good. some guys will then paint it so it doesn't corrode, or oil/grease it.
-double check the static timing, make sure the centrifugal/vacuum advance mechanisms in the ditributor are working properly while you are there.
-make sure you have the right spark plugs. check the reach (length) against the plugs that came out. possibly the wrong plugs, which would mean they hit the pistons. I have seen that before. usually the ground electrode is bent though.
keep us posted