Boss 302 into a 65?

That swap has definitely been done. You're correct - the 351 Cleveland/Clevor headers will hang too low on the 289/302 block. However, headers made for the '69-'70 Boss 302 *should* fit the '65.

Check over at www.fordpowertrain.com - I'm pretty sure they have Boss 302 4-into-one headers to fit the '65-'70 Mustang and Cougar.