Those 300's had a long run. You should be able to find one relatively easily! The market is pretty soft for them too - practically no one builds I6 racing engines.
1965/70 Galaxie/LTD VIN engine code (5th digit) B / This is the only I-6 available in these cars, the 300 wasn't offered.
Note that B is the VIN engine code for the 300 I-6 in 1965/89 F100/350's and Econolines. Y is the 300's VIN engine 1990/96.
1981/96: The VIN engine code is the 8th digit.
You cannot go by block casting numbers to ID the engine size since the 240 (cancelled after the 1974 model run ended) and the 300 share the same block.
Only the stroke is different: 240 = 3.18" / 300 = 3.98"