First I'll say: Very nice looking 65!!
Put a 318 in it! It's simple to do; it's 45 more cubic inches! Unless you tell them, 99.9% will never know. Keep the original 273 in a corner. A stock production line 318 short block with a cam change ( stock 340, Comp Magnum 270, EX268H, or something else in this range), 66 and later closed chamber 273 heads, or closed chamber 302's, eather with 1.88 intakes, and 1.6 exhausts, cut .030, Edelbrock Performer, or Holley Street Dominator intake( paint the intakes engine color), headers. This will make for a sweet, drive it any where, any time car. It would be capable of deep 14's to high 13's in the quater mile.
One other thing though, with your present tranny and converter, the hub on the converter is smaller OD than the ID of the crank register, you would need to have a small ring made to put in the end of the crank to stablize the converter, not a big deal, I had to do it to mine before I swaped out the tranny.
