Cheapest way to fix seats with tears and missing foam?

Gluing chunks into 50 year old foam isn't going to hold up for long.
If it's just the seat bottoms that need attention, go to the fabric store (JoAnne Fabrics, Hobby Lobby, etc.) and buy a roll of 4-6" high density foam. Cut it to the size of your seat bottom (actually, a little bit larger) and toss the old shredded foam. How do you cut foam that's that thick? Use an electric carving knife (yes, the same one you carve up your Thanksgiving turkey with. Just don't let the wife see you). Works slick.
Need more foam to do the seat backs too? Go to your local discount bedding outlet store and buy a 4" thick fullsize foam mattress pad. Start cutting. It's enough to do the whole seat and then some.
Now take your cheapo Mexican blanket seat covers and stretch them good and tight over the new foam, and hog ring them into place just like the original upholstery was. Yes, use hog rings. Don't even think about using the supplied velcro straps or elastic cinch straps, they won't last a week, if that long.
As for cutting it to fit your split bench, just get one that's already made for a split bench.
PU Leather 60/40 40/60 50/50 Split Rear Bench Cover for Auto Blue Black | eBay