Yes, no reason why you can't bolt up a 350 to the SM420. I've done it with mine and it works beautifully. But that puts the 350's valve covers and distributor too close to the firewall and you have to do some clearancing work. A lot of people go with automatics so that you can reposition the 350 further forward. But I personally prefer the SM420 because it is a kick to shift that 350 manually. And the power is wonderful.
But installing a 350 creates all sorts of other problems. The most obvious is you need a different steering system because there is NOT clearance with a 350.
The next problem is your firewall, which I've mentioned.
The next problem are the brakes. The lever arm on the engine side of the firewall won't clear a V8's exhaust unless you do some significant grinding right down to the bolt that links it to the pedal.
The next problem is cooling because the stock radiator won't be adequate. And a larger stock-style radiator that can handle a 350 costs around $700 bucks. But you can install a truck radiator right behind the grill with modifications.
The next problem is that the brakes are nowhere near good enough to stop an engine like the 350, only because it accelerates so much faster than it can possibly haul it down.
Given all the problems, had I had it to do over again, I would have redoubled my efforts to locate a 292!
But I've worked through the problems and the 350 is a nice match for this truck.
