Looks like I finally found the correct size. After removing the driver's seat I found a bolt installed by the factory for the lap portion of the belt. It was a 1x7/16x20 grade 8 bolt. I did not find a hole for the 5th belt. I still need to resolve that portion of the installation.
Well, I was planning to use grade 8 or better bolts which I think should be strong enough. I have not found any specific hardware for racing harnesses on line or local speed shops. I may have to start calling around if no one knows on this forum.
The calling should have happened BEFORE now!
All SRT10 Vipers are factory-built to accept SIX point harnesses. So you WONT find that front-center hole you seek, as was used in 92-02. SCCA obsoleted 5 points for racing in 2003, effective in 2/05. Dodge was clued-in and planned ahead due to the Comp Coupe.
Do you have snap-in anchors? Bolt-in flat anchors? Do you have an in-date harness?
If you dont know harnesses, especially Viper Harnesses, know your supplier.
If you think the bolts dont matter, or that simply drilling a hole in the floor pan will save your life, or maybe you are doing a car-show-photo-op harness, I guess it does not matter. Velcro would work for that...
If you plan to campaign the car in track events, there is a floor-mount crossbar than can make the (less safe) 5pt work for some sanctioning bodies.....