Two motorcycles. 350 pounds each, or 1,000 pounds each?
If 1,000 pounds each you are talking 2,000 pounds plus whatever support stuff you carry, so you're talking about an 8- to 10,000 pound gross vehicle weight.
Can you get a 235 or maybe a 261 (truck version of the 235) or a GMC 302 to move it at over 60 mph? Probably, though it might take some work, including finding the right combination of rear gears, transmission and tires.
If running an original type motor or preserving the originality of the truck is important to you, I say give it a try. You might be happy with the results.
But on the other hand, if you really want a V-8 go for it. If you are hiring a shop to do this, make sure you look at similar work they have done before. If you are doing it yourself try to look at a few others have done first.
And plan on upgrading the brakes just as much as you upgrade the ability to go.
