the 64-66 273 has a slightly different intake bolt pattern, close enough you can wallow out the holes and make it work, on the other hand an SP2P ain't gonna make any noticeable difference in MPG, that's the reason few were made and sold. The area of the curve where it helps is such a low RPM you would need an even lower stall speed converter than stock or a manual and highish gear that can keep it at low RPM cuz even a 2.76 with no OD is spinning pretty good at 75 MPH without some very tall tires. If I had a 273 and was running a carb for MPG set up I would run a stock 318 dual plane intake and pre emissions carter BBD carb.