Re: 6.2 vs 6.5 ?\'s (edited)
It depends on the year of the engine and the truck it came in.
Early 6.2L non-turbo with DB2-4126 to DB2-4878: light duty emissions C code engines ~ 160 hp and 280 lb-ft; heavy duty emission J code engines ~ 180 hp and 285 lb-ft. Add a Banks system with a Garrett T04B turbo, and power goes to about 205 hp and 390 lb-ft.
1993 3/4 & 1 ton 6.5TD with DB2-4911 ~ 190 hp and 380 lb-ft. You can tune this up to about 230 hp and 450 lb-ft.
1994-1996 3/4 & 1 ton 6.5TD with DS4 pumps were just about the same.
Late 1996 to 1998 3/4 & 1 ton with DS4-5288 pumps were 205 hp and 450 lb-ft.
1999-present Vans and Hummers with DS4-5521 pumps were about the same, but with blocks cast by IH, 19.5:1 compression, and bigger turbo.
Aside from making sure the engine is in sound condition, the basic formula for any good running mechanical 6.2/6.5 is a turbo (TE06H, T04B, GM8, etc.), DB2-4911, and timed based on the fuel and boost. If you're going for 14+ psi of regular boost, an intercooler is must.