I'm confused. If all you plan on using the truck for is a few short cruises and just an inline six, why change the original frame for something that will take a lot of work to make work.
The reason folks swap frames is to have IFS and V8 capability without so much work. But if you're keeping a six in there keep the frame and improve the front suspension with a dropped axle.
My advice (no charge) is to take your time and ask a bunch of questions like you seem to be doing. Also use the archives here, there's a ton of good information there.
