305 heads have a 58cc chamber. This is why they are not recommended to be used on 350s or 400s other than the fact that the small valves and ports just can't support the bigger cubes. With flat tops you would be over 11.5:1. Piston dish is usually 21cc so esimating your deck height and head gasket thickness as average for a never redecked 350 and you are at 10:1 right now. The heads are killing you. You are running out of flow long before you run out of engine.

Those 49cc heads will put you at 11.2:1. Try buying gas for that with an iron head. Those Edelbrock 60cc heads should net you 10:1 with the dished pistons and give you the flow you need. The Iron Eagle heads are also a great choice. The 64cc heads will land you around 9.6 but could be lowered to 9.5 with a thicker head gasket. This is as high as you will be able to run on pump gas and still run a decent distributor curve for power. (9.25 would be better. A rule of thumb for using aluminum heads is you need one more point of compression to make the same power as an iron head of the same cc. This is due to the much greater heat transfer out of the chamber into the cooling system of the aluminum head. I think you would end up with a stronger engine overall with the Iron Eagle 64cc heads and a milder cam that comes on around 1800-2000rpm .

If you insist on keeping the big cam buy the 60cc Edelbrocks and go for the 3000rpm stall converter or 3.73 and up (numerically) rear gears to get that heavy Poncho launched. Also put a 750 on that Air Gap so the new heads can breath.

Tom