Installing Lab-Shoulder Seatbelts from Andover

[Austin C10]

I'm having trouble installing some retracting lap shoulder seat belts from Andover (thru American Classic). They came with an L-bracket for the anchor on the retracting spool. The problem is that the anchor bolt, from the old seat belt, is right under the gas tank and I can't wedge the spool up under the cutout in the gas tank and still have the spool facing up. Any suggestions in getting the spool anchored?

Thanks!