Here's some really good news for you. In looking at the video, it appears that you have a pair of '416 heads bolted on. If you can confirm that (casting no. 14014416) you should remove and prepare them for re-installation (cleaned valve faces and seats, screwed studs, check guides, new seals, new springs & retainers) with .039 gaskets to bump your compression a bit. The '416s have smaller chambers and reasonably sized valves (for a 305) and with just a bit more squeeze can help improve performance, efficiency, and therefore, mileage. It would be best to get rid of the valve rotators on the exhaust valves - They are useless. Even the factory gave up on them because they do nothing to improve valve life. While the valves are out, machine the guide bosses for lip seals. You can get a set of lip seals for dirt. A set of GM factory titanium retainers, keepers, and new springs is only about $70. Shim the springs to the proper height and forget about valve problems ever again.
Mr. Gasket P/N 1075 studs:
Lightweight valve spring caps (to be used with valve spring kit P/N 12495494, and key kit P/N 12495503):
GM Performance Parts P/N 12495492 - $19.95
LT4 valve spring kit, P/N 12495494 - $36.95 :
10º Lightweight valve retainer locks (keepers), P/N 12495503 - $12.95 :
And a pair of ROL HG31010T head gaskets, or Victor-Reinz (factory) GM P/N 10105117 gaskets w/0.028" thickness for about $18.95 each should seal it up nicely. The GM gaskets are still multi-layer composite gaskets with sealing rings, so you won't suffer any of the problems associated with simple coated steel shim or copper gaskets used in street engines. Even though they have a 4.080" gasket bore, they'll seal the 305 heads well and provide just about the right quench area with a "normal" .025 deck height. They are an improvement over the stock 0.045 or generic Fel-Pro 0.049 gaskets. Besides, I've had a couple runs of bad luck with Fel-Pro head gaskets.
The coolant, valve grind gasket set, and oil/fluids will cost you almost as much as the good parts.