Walter;

...I wonder who told you seatbelt webbing works well...someone looking to sell you some? ...because I doubt it was someone who has actually installed some and driven thousands of miles with it without wondering why his back ached...webbing, whether seatbelt or other, has very little compliance or stretch, certainly much less than the OE rubber bands, and compliance is what's clearly called for here.

Don't let the foam of the cushions be the only thing (besides YOU) with give!
The designers who engineered those excellent seats went to great lengths to finally come up with the rubber suspension...and it was great...ask Irv G. who has spent more time in his seat than maybe the rest of us combined!...the only thing the original didn't have was a material which stood up as well as the rest of the car...so when repairing, I recommend against changing that from a functional stand point...and that's what an all-metal suspension with springs does...it's a permanent repair which does not change the original design intent...only the materials which execute it.

It's your car and you're free to install whatever you decide on, but in my career with vintage Volvos, I've tried everything from rope to plywood and I know what I settled on...

Maybe some others with hands-on (er... butt-on) experience can chime in here, because I'm probably looking to sell you something...

Cheers