To use stock cylinders and blaster pistons, you would have to bore your cylinders to 66mm.. Waste of money not to mention cylinder life.. Buy the long rod +4 crank, use 795 pistons and get porting done. When the time comes for the big bore.. Sell the cylinders and buy a cub/serval kit and use your bottom end.
A well ported and setup stock cylinder 4mil will rip. you probably wont even want a "big bore"... well.. probably will.. but at that point you'll go past a 421..