Either way, I'd pick up the '85 to keep the engine as a spare for yours.
The highest rated N/A 6.2 was:
155 HP @ 3600 RPM
250 ft. lb. TQ @ 2000 RPM
As mentioned the 6.5 N/A wasn't much different.
I think if you swapped everything you probably wouldn't notice too much seat of the pants difference, other than a fresher engine.
I'll bet the valve cover says GM Goodwrench, if it's a crate engine replacement.
Get the engine and keep it for when your brother mentions a swap to remove that gas hungry 454 from his 91, hold him ransom!
J
1982 C10 SCSWB 6.5TD, mods too extensive to list. (13.69 1/4 mile @94.6 MPH) RACE TRUCK
1982 C10 SCSWB 6.2NA, 2.73 700
1986 C10 SCLWB 6.2TD 3.73 700
1989 V20 SUB 6.2NA, 3.73 400
1994 G20 VAN 6.5NA, 3.42 60E
1994 K20LD ECSWB 6.5TD, 3.42 80E
1995 K20 SUB 6.5TD, Wrecked, ran into by stupid teen.
1995 C3500HD DRW 6.5TD, 12' Flatbed 5.13 80E
1995 C3500HD DRW 6.5TD, 18' Rollback Wrecker 4.63 80E
1994 C20HD ECLWB 6.5TD 3.73 80E Wifes Truck.
1995 C20LD ECSWB 6.5TD 3.73 80E
1995 K20LD SCLWB 6.5TD 3.73 80E
1996 K30 DRW 6.5TD 4.10 80E
1997 C10 Tahoe 2Door 2WD 5.7L to 6.5 Conversion Underway