The 300 is quite possibly the best engine Ford ever built. It responds well to mods and there are a lot of good hop up parts. I have warmed up a couple and actually have one built and another one in the waiting now.
I was not aware the 300 had early and late bell housings, learn something new everyday.
The 240 head is just a bit shorter and should use 240 push rods but has a little higher compression. On the 300 there is 4 different rocker stud variations, some adjustable and some not.
Clifford is a great source for parts but also Offenhauser still makes intakes. I have one that is a dual port and I believe it is still in production. I also have a 2 fours intake and there are several others made, even mechanical injection.
The later efi engines use a split exhaust manifold that is a nice upgrade. I have a couple sets of those and also a set of long tube headers but will probably opt for the manifolds when I install the built engine in a truck.
Also the engine will reward you greatly for any time you spend cleaning up the ports with a die grinder.
I like Comp cams and have run the 260 with great satisfaction.
I have strong plans for a 300 with small twin turbos running on propane.