383 Accusump port

I'm not sure how big you can go with the ports at the back of the block. If you're going to go larger than 1/4 NPT you should practice on a junk block to see how thick that area is. I'm pretty sure you can't put 1/2 NPT back there without ruining the block but you might be able to get away with 3/8 NPT. Check it on a junker before ruining your engine.