Step 1) Look for the date code. This is 4-characters, 1 letter followed by 3 digits. It may be 'inside' under the valve covers. The letter indicates which month they were cast with A being January, etc., followed by a 2 digit day, and a 1 digit year. The engine foundry 'switched over' and started making parts for the next model year about mid-year previous, so for model year 1969 your date code should either begin with an A-G and the last digit should be a 9, or it should be higher than 'G' and end with an 8.
You have to nail the model year first because they 'reused' the other casting numbers on the exhaust ports to mean different things in different years.
Step 2) For RA III 1969, the head casting should be either 48 (manual transmission car) or 62.
As for header gaskets, I've been using Remflex with very good results.