With regards to modifying the front end to accept a V-8 swap, you do not have to modify the frame or front suspension with a small block. If you intend to install a big block you might consider some front suspension upgrades to account for the additional weight of a big block.
My first truck was a 1966, C-10, LWB, Fleetside with a factory staight six and three speed on the column. I drove it like that until I had the funds and resources to install a small block 400. With the help of my father and my neighbor (a diesel mechanic) we were able to install the small block with minimal trouble.
Even at 16, I treated the truck like my best friend and loved every minute I spent working on it. I followed my first truck with a 1965, C-10, LWB, Fleetside that also had a factory straight six and a granny low 4-speed on the floor. I drove that truck for five years until it was stolen. With the aftermarket upgrades available for sixes, I would get the straight six running. This will require minimal costs and as others have stated give you a chance to see how your step-son will receive and treat the truck. If the 327 comes along later, the swap is rather simple and can be done as time and finances allow.
Best of luck with the project. Hats off to you and your son's father for taking a pro-active role in your son's interest in restoring the truck. Men of your character are the glue for young men heading in the right direction. Your son should cherish the experience for a long time.
