Hi,
for free play You have to shorten the 3rd and 4th contact of the credit relay in the backbox. See the black wires on my image. Just ignore the yellow ones.
You can install a switch there to toggle free play.
Just make sure, You remove any other helpers and adjust the relay switches properly, If You want to add credits.
The playfield glass ist standard size (21" x 43"x 3/16") tempered glass.
Regards
Michael