I'd keep and rebuild the original engine before I'd put a Chebby engine in there. The original Y block V8's are neat motors. And the 215 OHV sixes are neat motors too. A decent Y block will be more than enough to power that truck. It will get it as fast as you want to go in a heavy vehicle with a 60 year old suspension design and drum brakes on all 4 wheels, anyway.

GM motors are actually pretty common in these trucks. My dad has a '48 with a Nova IFS and 350/350 in it. Most people just weld on the front clip from a Camaro or Nova, as it has SBC motor mounts, IFS, and usually disc brakes too. Back in the day it was harder to swap Ford motors because until the Mustang II came out they didn't really have a motor/suspension that came as one unit like Chevy did.