[Big J'72 GMC]

I got a '72 GMC I'm workin on, tryin to get done this summer, all I have for a drivetrain is my good ol 12 bolt, or engine or tranny, still have the stock driveshaft kickin around to, I'm not all the knowledgeable whe it comes to engine an tranny combos an I was just wondering if you guys culd tell me what my best bet is to get a smallblock an matching 4 or 5 speed fairly cheap, thanx in advance