The 273 had a different intake to head bolt diameter and angle than the 66 and later 273-318 etc... The early style had 5/16" bolt diameter where later style is 3/8" diameter intake bolts...
In other words, the 64 and 65 273 heads are different than the rest of the other years... If you still have the original heads, then you may want to look for the early style bolt pattern intake...
Or if it has 66 and later heads, or swap some 66 and later heads on it, then you can use any standard intake for the LA style small block...
Look at the casting numbers on the heads, then look them up on this chart:
