not positive but i think the big trucks have all different sheet metal ahead of the cab. longer and wider. a factory assembly manual would help you out as they have factory frame and body dimensions.
or just wing it with what you have.
gmupfitter will have frame and body dimensions for the newer truck frame under the body builders section
there is a fella on the forum who put an AD burb onto a trailblazer frame and floor/firewall and he used the big truck fenders blended with the small truck fenders i think.
check his build out here, the fenders are on about page 8 I think
https://67-72chevytrucks.com/vboard/...=348244&page=8