Welcome to the forum. Your new truck looks straight and solid. Even the tailgate appears to be in great shape. Here are my thoughts on the V6.
The GMC V6 was probably the most over engineered automotive engine ever built. For uniqueness, torque, and longevity the GMC V6 is in a class of its own. It is great for hauling heavy loads, but not very efficient in gas mileage. It also weighs about 800 lbs which sits right on top of the steering, and will likely cost about twice as much to rebuild as a small block Chevy. A rebuild however will probably last another 40 years, and if you attend car shows you’ll get a lot of attention as these engines are rarely seen.
If you decide to keep the V6, do yourself a favor and join the Jolly GMC site if you haven’t already. They have the corner on V6 expertise and can no doubt help with advice and parts. Just don't ask about engine swaps on their site. They are V6 enthusiasts exclusively.
http://www.6066gmcguy.org/Whether to keep this engine really depends on personal preference and how you want to use the truck. One of my sons built a 401 Buick. It’s a sweet engine, but cost him over $3,000 (ten years ago) after a lot of searching for some common but hard to find parts. Certainly a 350 Chevy would have been easier and less expensive, but no matter if it’s an Oldsmobile, Chrysler hemi, or GMC V6 – if it’s what you love it’s worth the cost and effort to have something different.
I analyzed this decision myself about 20 years and went the other direction, replacing the V6 with a 350. My engine had cracked valves but continued to run reasonably well. I checked with several reputable shops I trusted, and was told they weren’t interested in rebuilding it at all. Two that were willing wanted over $2500 for the effort. At the time a brand new 350 was just over $1,000. Since I wanted a high performance long distance hot rod commuter, the decision was rather easy. Personally I’ve never regretted the decision to go with a SBC, but I also enjoy studying a restored truck with one of these monsters installed.
A small block V8 will fit right in, but I believe you’ll need the motor mount towers from a similar year Chevy. In my 1960 the V6 motor mount brackets were welded to the frame. I’m not familiar with how GMC did this in 1963.
Good luck on whatever you decide.
