If there is a 6.2 in it you may not be missing anything. The 6.2 can replace the 6.5 without any real difference.
Now if they changed it from EFI to Manual fuel injection that is a bit more complicated.
Post some pic's for disscussion
90 Chev 3500 c/c 4x4,6.2na,400 auto,4:10 gears.DSG Timing gears,main girdle, isspro tach, pyro,boost,oil and trany temp.Dual Tstats, High volume peninsular pump,on shelf, Custom turbo and intercooler 85%complete. Change of plans for the dually, it's going to get a Cummins. Both trucks are Blue 90 4x4 crews