i bought from Andover for the front, and now rear. good belts good service.
My 63 SF vert had plates and bolt holes only for the front. The Andover kit comes with plates to reinforce the outside, after you drill the hole. I think it is essential to have at least lap belts in the back of a convertible. Eventually I shall swap the front seats for ones with integral shoulder harnesses. 60s cars had NO impact protection except for the optional padded dash. The first mandatory safety improvements started in 68, along with the requirement for no tune up for 50,000 miles, etc., which is the reason that US cars had electronic ignitions and fuel injection long before Europe. i was in the USAF in the UK then, owned a smashing Lotus Elan: 1558cc, rev governor at 6500rpm, =111mph! I remember the drivers at the Lotus factory circling...well, triangulating...on the old WWII RAF station runways to accumulate the 50,000 for certification.