You can use the stock crank and pistons, but you are assuming a couple of things here:
1) They (rods & pistons) are up to handling the new power (as memory serves me, stock rods are good up to about 400 hp or so.
2) the 425 hp you make is still in the 'safe' rpm range for your engine with the stock bottom end. If the 425 hp comes in at say 6100 rpm like they say it does, you should be alright, at least for a period of time. How worn are the rings and bearings? Grumpyvette or one of the other experienced guys would be able to tell you best.
Keep in mind that the Systemax kit probably works 'best' with forged pistons and quality rods, because Holley owns Lunati and they want you to buy the stuff. Seriously, if you bottom end is new like the top end, then you'll have a motor that will live longer, especially with drag strip abuse and the like.
The Systemax kit is good bang for the buck IMO. Even if you didn't keep the intake manifold, you could always sell it and recoup at least $100 on ebay for it--seems like many people pay too much for junk, so I wonder what they will pay for a new quality piece?
Davy