As Brad posted, the bigger questions would be the strength and positioning of your tow set-up, your rear springs (should be OK), and your brakes. Your rear ratio is relatively high but you have the SM420 1st gear to get you moving.
The 270 engine was used in larger GMC trucks (at least up to 2-ton), as was the SM420. Of course they will "cut it".
On what types of roads will you be driving and at what top speed?
