Hi kcwis:
Yeah, if you want to put on towing mirrors, this is the time to make mods to your doors and make sure you got the holes in the right places. If you are using aftermarket doors they are most likely of thinner gauge than the factory doors and fo sho need reinforcement. you can use welding or adhesive. If you use adhesive, best to use reinforcing material that is steel instead of aluminum so you dont have differential expansion...use a high quality structural adhesive such as 3M #2216. The doors are especially sensitive to vibration under the quarter window, so if you have a mirror attachment that is there, you should reinforce this area. You can make your reinforcement plates thin sheet steel if you want, the stiffening can be accomplished by having a bent lip on the reinforcement plate. I personally like the use riv-nuts as the permanent mounted nut inside the doors for ease of installation and removal of mirrors. You can make the riv-nuts attach to (or through) the reinforcement plates. I think McMaster-Carr has a selection of these and the tool required. Whatever you do for reinforcement, check and re-check your design to make sure it does not interfere with the window mechanisms and door lock!
Good Luck,
Henry