Edelbrock RPM Head compatability issues.

Is there a way to find out if the pistons I have will protude outta the block or will they deffo stick out?

I would start by finding the "Compression Height" of the pistons. That is the distance between the center of the wrist pin and the top of the piston. If it has a stock 340 Compression Height it will stick out of the block. How much is a different story because block Deck Heights vary from one engine to another even within the same year engine block. The factory was sloppy when it comes to those specs.