Given that you seem to be building a car which is a curious blend of perfect restoration with the fastidious use of NOS parts, yet with a modern engine offering far more performance than original (and, doubtless, modern brakes to boot), I'd question the use of lap belts - particularly in the land of the "utes"! What's wrong with using a three-point harness? They were made and fitted in period, you know .
I'm not saying you're entirely mad, but three point harnesses were offered in period and my car has had them from new. Nothing cool or 'old school' about head and chest injuries if you lose it in a corner, or some idiot in a truck rams you. There are plenty of companies out there (such as Securon) who will make you a new and safe set of webbing for your car, using materials which are age-appropriate.
Incidentally, I was under the impression that a cable-operated revcounter could not be used with a twinspark 'implant', so it would be pointless to deprive another car (with the correct engine) of a genuine NOS item. You'd be better off finding a cosmetically good used item and stripping it out to install the necessary electronic innards, perhaps?