1/2 tonners in the '41 thru '46 range are a popular breed, in both the Chevrolet and GMC camps.. many parts are interchangable between the two, so keep that in mind when looking for parts.. they don't always have to be 'GMC' to fit.
If the fenders are rotted out that bad, i'd go with either new reproduction fenders, or try and find some better condition old ones. Fenders full of patches and bondo aren't going to help resale value at all, and unless you're skilled in that area, would probably cost more to pay someone to fix than just buying the repros.
Of course, a vehicle with original or 'period correct' metal is worth more than one made of fiberglass.. but only if that metal is solid and bondo free.
How the engine being changed effects value can go either way.. folks seem to be split about 50/50 on the subject.. some are staunch 'keep it 100% stock original' guys.. while others are all about slapping a small block V8 in there.
Modest engine updates.. like replacing the engine with a full pressure 261 or 235 from a 50's era GM product tend to ruffle the least feathers in the 'purist' camp.. the fact that the engine was replaced in the 50's makes it less harsh of a concept to the purists out there.
1/2 tons are worth the most money when they have the original factory bed, fenders, and running boards on them. While we can all stand back and admire grandpas ingenuity at putting a grain bed on a 1/2 ton truck in the 50's.. for todays truck collector, the factory box with fenders is what they gravitate to.
In the end, it's all about what YOU want to do with your truck. Do to it what YOU like, and what makes YOU happy.
If your thinking of fixing it up for resale.. don't bother.. because whatever you do to it will be wrong to somebody out there.. just get the engine running, make it go forward and backward, then stick it on eBay.
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