Disclaimer- this is based on the measurements that I took with my 87 442. Your shiit might be a tad different. Take that into account. In other words, you can't hold me responsible for measuring stuff.
Most rolls of material are around 57-60" wide. It also depends on whether your pattern goes "across the roll" or "up the roll" as to how much you may need. For example, most of your "ribbed" looking material goes UP the roll, meaning that you would have to lay it across the seat sideways. You would only have up to the width of the material on the seat back or seat bottom. If you have no pattern, you can likely go either way.
I did write down the measurements of the seating surface of my 87 442 Supreme non-recliners. Can't think that any other similar G-body wouldn't be approximately the same. It's about 22" at it's widest, and approximately 22" front to back. It tapers here and there, but that's the widest point you'll need. So say a 2 foot by 2 foot piece for each seat bottom seating surface area. That material is up the roll, so you would still have 3 feet to use on the backing. It rolls over the top, but it also depends on if you have another type of material involved, like the 87 Cutlass does. I think Montes have headrests? So that saves a bit there. So you're basically looking at 2 yards of material to make sure you have enough just for the seating and the backs. 87 Supreme non recliners (and probably recliners too) can get by with 1 yard of material per bucket. You should have enough for the top of the headrest area left over as it's around 26 inches or so and around 23" wide. 87 owners will have to get the different material [smooth] for the headrest area on the high back)If you have the same material on the sides of the seats, you'll have to account for that measurement as well. If it's vinyl, obviously that is a tad easier to find.
Back seat seating area is approx 55" wide, by approx. 19" front to back (a couple of inches added for going under the top seat section), and around 26" tall (small wrap around at top). There's about a foot section width in the middle of the smooth material. This makes it about total 5 1/2 feet in length and the width should take care of both top and bottoms of the rear seat. So 2 yards here SHOULD be enough to do the rear seat.
You COULD squeeze by with 3.5 yards I'm thinking. But if you want to ensure you have enough to spare, you're looking at approximately 4 yards front/back buckets. You'll definitely need 4 yards if you're benching it front and rear I'm betting.