The tailcone is from the current silver comet kit.
The BT-80 is 7.5 inch long.
You could use the nose cone from the silver comet but the
detail would need to be  sanded off and any "Gaps" filled
or patched, so the nose cone from the Phoenix would be easier.
Make the fins from 3/32nd inch balsa ( or plywood for more strength).
The BT-50 motor tube is 7 3/4 inches long.