I have a good friend that does Street Rod upholstery, and it seems to me that he did an early Suburban headliner a few years back.
I believe he used the original wavy hardboard as a template, and the new board was SHEET STYRENE.
Available through most custom upholstery shops in a variety of thicknesses. last 4'X8'X1/8" sheet I bought was around $40.00.
He "backsided" it with a good quality "peel and stick" sound deadener, (the actual roof) and covered the outside with the same material found in late model GM cars and trucks.
It's relatively cheap, and widely available. In all GM colors, too!
It looks nice with the stock seats, or with late model upgraded seats.
The"bows" I believe, were custom brushed stainless with Oak inserts.
You might consider adding a couple of extra bows, if there are additional inner cross braces to attach to, as these headliners are heavy and not supported too well originally.
You might check some of the later GM vans for sunvisors in the same matching material, too!
Good luck!
Mike
Oh, BTW, I highly recommend some sort of insulation between the roof and 'liner. COOL CAR (KOOLMAT?) makes some good stuff.
It will definitely make your A/C work less!!