First off, Welcome to FABO! :welcome:
Now as far as heads go:
The 64 - 65 273 heads had a unique intake bolt angle and smaller 5/16" bolts, so you want to stay away from those, as intake manifolds are harder to find...
The 66 and later small blocks had the larger 3/8" intake bolts at the "standard" angle...
The 66 - 67 273/318 heads had the smaller closed chamber heads, which are the smallest (and therefore higher compression)...
If you swap 68 and later heads on it, you may loose some compression... I have a set of 67 273/318 heads ending in casting number 920 and I measured them at 63 cc which the book lists is supposed to be 57 cc's. Chrysler ran the heads "Fat" with a little extra meat, so the book numbers for combustion chamber volume will not match...
My favorite intake for the 273/318 heads is the obsolete Edelbrock LD4B intake. It's a dual plane 4 bbl intake and is great for street/strip performance. However you need to relocate the coil as there are not any bosses on the intake to put it in the factory position.
Next I would recommend the Wiand stealth intake which is currently available.
Weiand 8022WND Weiand Stealth™ Intake - Chrysler Small Block V8 - Holley Performance Products
Which will go perfect with this Holley 80457 carb, which is a 600 CFM square bore carb that matches the wiand stealth intake and calibrated for a late 60's v-8 and comes with electric choke:
Holley 4160 Aluminum Street Carburetor
And I like this carb gasket as it has the center divider to match the dual plane intake and also comes with mounting studs....
Edelbrock Heat Insulator Gasket
You need to be careful if you try to put larger valves in the heads, as the bore on the 273 is pretty small and can restrict/shroud the flow, or you may have to "Notch" the bores for better valve clearance.
That will be a good start.
