Yeah I'd use the spacer. But make sure it's on straight and doesn't wobble. That wobble can really kill the bearings in the water pump. You can get up to a 2.5" spacer, but that's getting pretty long. Depends on where your 350 is.

Most people move the 350 a few inches forward if they're using an automatic transmission just because you get some much needed clearance with the firewall. If you do that, you might not need a spacer to get the fan close to the radiator. I put my 350 in the stock location so that I could still use my 4 speed transmission. But that put the engine so far back that I needed a spacer to move the fan more forward.

I've abandoned my stock radiator for a modern truck crossflow radiator mounted just behind the front grill. It worked great until my NAPA water pump came apart and the fan tried to fly through the radiator. But they're doing the right thing and paying for the damage. This time, I'm going to put a fan guard behind my new radiator, even though a freak accident like that should never happen.

52 GMC 3/4 ton pickup
68 Big Block Vette
68 455 Firebird