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.