I may have another option. I have a '68 Camaro Convertible, C4 IRS, Art Morrison C5 sub-frame and C6 LS2/T56. Primarily for saftey reasons I wanted the shoulder haraness. My daily driver is an '04 CTS
I bought some Sebring Convertible seats that have the integral seat belt but also articulate and slide forward to allow easy access to the rear.
The car's at a shop so I don't have pictures but here's a link to a guy that put them into a Corvair and how to do it (
http://www.corvairkid.com/sebring.htm). I have an extra wiring harness if you're intersted in this path.
Good Luck!