The 6.5L from GEP would be by far the easiest swap. The other swap that would still be easier than the 6600 Duramax would be the 88-93 6BT VE Cummins or 94-97 P-pump Cummins. All the parts needed to do the conversion such as engine mounts and pulleys to put into a Chevy are available. However, automatic transmissions don't seem to hold up too long behind a 6BT Cummins especially when power output is increased from stock or lots of money is spent to build up the automatic transmission. For this reason, I chose manuals on my Cummins conversions.
1990 ¾ ton 4x4 Chevy Suburban
-Cummins Diesel - 12 valve - factory rebuilt
-6 speed bullet proof manual transmission - NV5600
-Gear Vendors Overdrive
-Upgraded Holset HX-35 turbo
-NP205 iron transfer case
-3.73 gears
1982 ½ ton Chevy Suburban
-6.2L diesel - high nickle crack free 1982 block
-Stans headers
-Ported heads
-Timing gear
-4 speed automatic
-3.08 gears
-30 mpg on freeway