I started with a 300 High Output Plenum. This & the LH platform 3.5L have the same Plenums... It's just a matter of locating one at a reasonable costs. You could always start with the one on your Prowler but, you would be out of commission for three to four weeks getting the entire project done... I started by using 180 grit sandpaper, and went to 80 & ultimately to 40 (very course). It took the 40 grit to really cut the raised portions down. I finished the whole piece by hand using the 180 and finished it with 320 for final sanding before priming...