Are you sure you got the right windshield. I can't remember for sure but I think its a '98 Mustang convertible??
Should be a drop in fit. Use lots of thick .25-.5" weather strip to support it. Key with glass is a "snug fit" not too tight or the chasis flex will crack it first run.
Other option is Lexan, but you have to make center supports. I got the last Lexan from Griggs Racing in Sonoma CA.
cheers,
Kevin
'08 GT3 Cup
GTS for sale