I think there is two different discussions going on here, the original fiberglass tops and the reproduction fiberglass tops. I have restored at least one MS80 with the reproduction fiberglass. Cant remember if it was Pergyl or GPH that sold them.
Basically, you need to have some bodywork skills to use them. I did a little trimming and had to fill the parting line (down the middle) with body filler. I vaguely remember there being some issues with the radius of the corners and how they met with the metal side panels. It all worked out, just took some massaging before jumping into prepping for paint. My advice would be to test fit it on the pump and fix and issues before proceeding further into the normal painting steps.
All in all, I was happy with the results.
Alternatively, you can get a tall metal blank top and do like I did recently on this pump. I'm still considering making a kit for it. Scroll down to April 24th posting. https://www.facebook.com/roadrash
(Oops, ignore my comments on working with the reproduction tops. GPH seems to be selling something new, I think my repros came from Pergyl, although I think the advice in using would be similiar).