Later A-body seats used a loose woven poly material that superceeded the burlap (which is organic and eventually wears out and rots). If you are going to replace the burlap, you might check it out. I've see 72 or 73 seats that used it originally and it looks like new 50 years later. I just redid my 70 Dart buckets and got foam from the only source - PG Classic. The difference in the density of the foam (how much resistance to compression) was remarkable. The seats were suddenly firm and supportive and much better on long days on the road, so if you are going to drive that car, it might be worth it.
