Yeah, I would not put the crossovers in the doors either. I run the CSII mounts with aftermarket speakers in them, and I put the driver's side crossover behind the radio with Velcro tape and the passenger's side behind the glovebox. Both are mounted to the air conditioning system and cause no clearance issues. The Velcro I used is an industrial type sold by Wal Mart in the fabric center. The speakers I used were not very expensive ( Phoenix Gold 5.25 separates I got off E-Bay for $65 with shipping), but they make a HUGE difference in the front sound stage over the stock 3.5's. They are run of a 75 watt per channel 4 channel amp mounted in the trunk. I did have to cut some metal out of the door to get them to fit, even with the stock mounts. I think GM used a different door stamping in CSII Regals to fit the speakers, because they will not fit without cutting. You have to be careful when cutting that you do not cut too much. This area is critical for support of the power window motor so you need to leave enough behind that the door won't stress crack.
I also used baffles to keep the back of the speakers dry. The speakers did not like being sealed though and would make really bad sounds when I turned up the volume. I solved this by cutting a small port in the bottom of the baffle. There was still some bad sound coming from the passenger's side though, and I had to cut the hole in the door a little larger to let it breathe properly. This may not be the case with your woofers, but it was with mine.