Yeah... those Mustang trunks have a small opening... I had to build 2 single ported boxes and join them together last time I put a pair of 12's in a Mustang... D*mn thing hammered tho... that XL moves some air and the port length will be longer due to the area needed for your power... IDK... I played with it and a single 5" round looks the best, 24" long... the multiple 4" aero is kinda long and the 6" has LENGTH.. if it was mine I'd do a single 5" round, flared a bit and side firing with the sub firing to the rear... it would be an experiment for me as I haven't done one in a Mustang as of yet... 8.75 motor width and 9" depth... thats a biggun'... Im not gonna be here for a few days until the weekend... but Im sure I can try to help when I get time... wanna give it a go?.. Whats your measurement between the wheelwells?