Installing aftermarket sheet metal 64 c10

[Lunnb]

I’m looking for any tips, tricks ,etc for installing aftermarket sheet metal . My original fenders , inner fenders , core support and grill support were beyond resurrection so I purchased everything from the aftermarket . Individually, the pieces fit pretty well but the final fitment with gaps and body lines will need some tweaking. After elongating bolt holes and gentle persuasion, fitment is better but no where close to where it needs to be . Is the factory assembly manual any help with this task ?
Any suggestions will be greatly appreciated .