I am far from knowing it all but I have never heard of a medium block! I know of big blocks and small blocks. I agree with frost and think the .21 is a big block. Either way the .21 would take a lot more to put in then a small block. I would get a OS .18 CV-RX and if you want all out FAST and see how well your truck is made get the highest HP small block make the OS .18 TZ. I hope to be getting the TZ with in the next couple of months.