I RUN A PRO FORM TRACKSIDE SPACER ON MY 406 SBC MAKING 600 HP.ITS PLASTIC AND HAS THREE INSERTS A 4 HOLE,4 HOLE TAPER,AND OPEN.THESE ARE AWESOME PIECES YOU CAN ACTUALLY GET A SMALL TUNE DIFFERENCE AT THE TRACK BY CHANGING THE INSERT.THEY RUN ABOUT 55.00 BUCKS.