Thought I'd share my experience with my 1979 Trillium
4500 frame bolts. Many months ago I replaced most of the 8 body-to-frame bolts. All are located inside benches, four in front and four in the rear. I used Superstrut 3/8" square washers similar size/thickness to original floor washers, 2" x 3/8" stainless bolts, with stainless washer and nylock nut under the frame. Tightened just snug to avoid damaging FG body.
Hardest part is removing the old rusted carriage bolts. Cutting off with a grinder works.
Edit: Actually its a 1978!