The 300 inline six is a good truck engine. It has a lot of torque, and to get the same bore and stroke in a V8, you would have a 410 engine. (390 FE, with a 428 Crankshaft)
That engine makes a good engine for a Ford Truck, but you do not have one.
It was pointed out to swap a 400m V8, it would be easiest to have a donor truck.
If it was my truck, I would rebuild the 300. More than enough power, and potential for reasonable gas mileage.