6.2's had a higher compression ratio. I think it was due to piston height. 6.5 and 6.2 pistons are different as 6.5 pistons are coated. Rest of the 6.5 short block is simply a bored out 6.2. The heads are different - precups and injector angle.
You can use 6.2 heads with a 6.5 turbo setup. Wrap the manifolds and add 2 exaust gaskets for injector clearance.
Oil pans can be different and rear main seals can be different designs including crankshaft. Oil cooler lines can be different sizes in the block. Turbo oil feed is a easy challenge.
There isn't more than 3 MPH difference at 65 MPH dropping in a 6.2 longblock with a 6.2 IP in a 1993 turbo truck like mine. Little more exaust haze from the precups and heads.
