318 crank pulley correct size??

That counterweight on your 3 groove pulley is held in place with a rubber insert. It is almost like some sort of damper ring but for what purpose, I don't know. I wonder if it is off of another engine, or perhaps a supplementary vibration damper for the AC compressor vibrations.

You may want to look at CVF Racing if you are OK with aluminum pulleys. They have quite a variety.

Yeah on FBBO it's 'big block or no block' LOL.