G'day Steve,
This job is one of the type that can really try your patience but when you succeed you'll be made up. I did my tin top this year, I was dreading doing it but the mission was accomplished.
I guess with your bus having a pop top this will possibly create more probs than a tip top but I'd say it's worth having a go. You also need to employ a friendly octopus to hold things in place whilst you stretch, tug and glue the thing in place.
I paid about £120 for the liner from DB Car Trim in Blackpool and was quoted about £300 by some other dude to fit it hence why I did it myself.
I started by clamping the liner at the rear window frame and pulling it all the way forward, inserting the roof bows as I went until I got to the front where again I clamped it in place. Make sure the liner is aligned down the centre of the bus. Now then, the trick with this job is to keep the liner nice and warm. This helps you to stretch the liner into position without it tearing, it also helps it to form round any complex curves but the great thing is when it cools down it pulls in nice and taught. I used an electric heater and as I did mine in the summer I sweated my bollox off. I'd also recommend a good spray adhesive.
So starting from the back I simply (ensuring all the while that the liner was nice and warm) pulled a 2' length of liner taught to see how it looked, adjusting the tension to get rid of any creases before clamping it to the rear side window frames. Then I sprayed glue on the areas to be bonded (just inside the window frame) before firmly pressing the liner in place and waited for the glue to do it's job. I worked in this way all down the bus to the front of the bus. If you get any creases don't be afraid to unstick it, heat it and try again.
A few points to note are that you need to trim the vertical areas of the window frames and the sills before you fit the headliner, also, when I did mine all the windows were out which allows you to easily fold the liner into window apertures. I've got some before and after pics of mine somewhere, when I suss out how to load them I'll post 'em on here.
Go for it.
Cheers
Ian