[Chuck Moreland]

I always advise that you put in the largest cooler that will fit, doubly true with a 3.6. A small inexpensive cooler wont seem so cheap when you have to replace it with a larger one to get more capacity. A setrab std-172 would be a good choice.

Mount it to the tub. You don't want all that weight being supported by a fiberglass bumper. It will crack.

For the cooler to be effective, you MUST provide a good exit path for the airflow behind the cooler. The Ruf bumper has more room than most, so you may get away with mounting as far forward as possible. However that leaves cooler more prone to damage. See Thom Fitspatrick's recent curb incident that destroyed his cooler.

The better choice is to notch the front of the tub behind the cooler, providing an exit duct ala factory '73 RS style. Then you can mount the cooler against the front of the tub.