I use fusible batts on huge quilts - on some of the old quilt tops I've bought, it's been a huge blessing to help keep that fabric under control until I can get it "nailed down" with quilting.
Pressing the back of the quilt, first, and getting it to lie flat will be a big help.
It IS a giant pain in the neck to get it basted, but I put together a huge 8-foot basting table with sawhorses to support it, 2x4's across those, then three more 2x4's across those to support the OSB plywood.
I try to baste a lot of quilts all at one time because the basting table is cumbersome to set up and it takes up half the living room. But, before the table, I was crawling around on the floor, basting quilts - even the fused ones!
You will need to fuse the top, then turn it over and fuse the backing to the batt. I always add pins or use the tagging gun to re-inforce the fusing, especially along the edges of the quilt. I pull the excess backing around to the front, roll/fold it and pin it like a protective envelope over the edge, so that the fraying parts aren't exposed to any more abrasion.
It's a lot of work, but it's worth it to save one of these old tops.
I would quilt it pretty heavily - not more than two inches apart, even though the batt allows up to five inches, if I remember correctly. All that extra stitching will help support the old fabric with the new, strong backing. Best of luck!