Looking for examples of 3 Point Seatbelts in 1964 Valiant

Oops, sorry. Links fixed. The now-working 2nd link, and the link it contains, addresses the 3-point belt issue and the seatback-latch issue (and brings along the equally-important head restraints). The 1st link (with its own links to Securon belts bought from SVC) will serve for your rear belts.

chiefly concerned with getting 3 points in the back bench as that is where my kids will ride.

Best revise that thinking, because your kids would probably like to keep having parents.