Thanks to all so far! I just checked FSM, which states that both '74 E & B body take 45 oz. of 12. So, 3, 12 oz. cans of 134 = 36 oz. which is 80% of 45 oz.

I know it is also important to keep an eye on the system pressure, but 3 cans of 134 would get you 80% of the R12 specs. Sound right as a rule of thumb?