3998993 is from 72 or 73 307 or 350. 75 cc chambers.
This number will tell you what year and what moter the heads will fit.There are books put out for this purpose.
Actual olds heads casting numbers are underneath the valve covers
they came with "186" casting number heads.........
Aluminum.
The combustion chamber size of the heads with the casting number 376450 is 60cc. It is possible to install larger valves.
The chambers are 60cc. The heads are from 77-78 307 engine.
If it has a 5.3 engine, first have the cylinder heads pressure tested. Heads with a casting # of 706 like to crack in the number 1 head bolt hole. If this happens, replace head.
The easy way is to look for a casting number on the block and try googling it. The harder way is if there are no pistons or heads on it you can measure the width and depth of a cylinder and then multiply those numbers together to get the cylinder size, then multiply that by the total number of cylinders to get the overall cubic inches.
Find the casting number on them... type "GM (insert casting number)" into a search engine and you should find what you're looking for.
There 194 heads 1970 to 1980 heavy cast iron heads
These should be double hump heads. They are very sought after heads.
Heads: Casting Number 3904390 - 1966-68 427, Intake 2.06, Exhaust 1.72, Closed Chamber, 98.4 CC.