you'll have to pry the cap off the rear of the old master.

make sure you are using a master with the same depth hole for the pushrod as you had before.

pic is from pozzi\'s motorsports

i think there are insructions in the service manual at 1968firebird.org
they menion that the cap will no longer fit tightly but the firewall will hold it in place.
there might actually be a retainer clip for it rather than prying, i dont know.