The "315" heads are 64-65. They have a small, closed combustion chamber so they give higher compression and are desirable in that respect. The '64-'65, 4 barrel intake manifolds were cast iron, early design single plane and are heavy and hard to find.
The '66 and later heads have larger combustion chambers but intake manifolds are easy to find and there are several designs and brands to choose from. To me, the "Offy" intake manifold mounting holes look like later model size and angle, despite what the box says.
So, you have started collecting parts to give you options to go different directions. Now, you need to decide what you want to build next to suite your style of driving.