1999 cadillac deville rear deck speakers

Hi I am wondering if anyone can tell me I am trying to replace the rear deck speakers in my 99 Deville and stupidly I listened to what I found online instead of taking 2 seconds to check in my trunk at the rear speakers and bought some 6.5 JL audio speakers and was wondering if since the back is 6x9s if it would be worth it to just alter the mounting holes with some plywood to fit the 6.5s in the 6x9 holes or just wait and get some 6x9s when I get the extra money thanks for any help in advance