I'm pretty sure ChevyDuty has the crossmembers you are looking for. If you don't need the power of the big block I'd go with a 350.
Lots of things to think about when choosing performance options. I like a reliable driver that can get out of its own way. Carburated V-8 does that quite well. If you just want a clean running motor I'd check the Chevy dealer for a Targetmaster. Won't cost much and should run solid for years. I have two of them.
Are you planning to run an automatic? What do you have in mind for a year end? If its stock your truck has a torque tube rear under it now.
