Rear Lap belt mounting for 1966 Convertible

[mitten]

Apologies if this subject has been done to death - for some reason I can't search past Page 1 using the search engine.

I have bought some Scott Drake lap belts for my '66 convertible to fit in the rear. The car has correctly sized threaded holes in the floor pan so I'm guessing there may have originally been factory belts ( nothing fitted at the moment however).
The hardware that came with the new belts includes nuts for the bolts, however accessing the underside to fit the nuts would be quite awkward, particularly the side holes.

My question is: Safety wise, as the existing holes are threaded, do I even need to secure the nuts from the underside ? I would use the large washers to spread the load on the interior side.
I'm wondering if the nuts are needed for when you have to drill out mounting holes yourself that wouldn't be threaded.

I did see a video online where they just screwed the bolts in with no mention of underside fixing, but would rather check here with someone who knows

Many thanks

[richard]

The threaded mounting plates should be there from the factory and I guess if the kit is from Scott drake then the fasteners should be the right thread (and not a universal metric nut and bolt set). You could pull one of the front belt bolts to compare.
After 50 years with nothing in them ideally a tap run through or putting a junior hacksaw cut across the first couple of bolt threads to clean them up might be helpful.

[pmustang]

Nut not needed. Just screw bolt into the threaded hole in the plate

large washer was for when there is no threaded plate and spreads the lord from underneath. I always had to cut the washer in two edges to get it to seat between the inner wheelhouse and frame rail where the belt attaches to the body in those situations

[mitten]

Thanks for the advice, that simplifies things greatly & allays my safety concerns. I did think that the washer was a bit on the large side Peter !!
Tapping out the thread sounds like a good idea also so thanks again both.

Cheers, D