I used the XR280HR in my budget pump gas magnum build I just finished in December. I used stock 318 heads. I run 1.6 chevy rockers and bought the comp cams cutter to cut the guides down. Had to cut them a lot to get good retainer to guide clearance. I used the 2.2 retainers and mopar P4876062 springs.

My setup is close to yours at 3300lbs and 3.55 gears and it shows some promise as it run 11.38 @ 118 through full exhaust and a LOT of ET left in it.