BSA fork tubes 1964

there seems to be two lengths of fork tubes for BSA's circa 1964. I bought a set which is approx 22 3/8 long. The ones I have are a bit shorter by almost 1 1/2" . what model do the short ones fit? I have a 1964 thunderbolt which is in the cross over years. What happens if I put the longer ones in and make a spacer for the springs?