Bob, It sounds like you are building what I did in KY. I used the aluminum with insulation under it. Concrete floor with freeze proof water faucet and a floor drain, 50 amp service. I put 10X10 doors in with a walk in door BUT the “barn” was a pull through so I never had to back in. I had a 24 foot wide X 29 deep attached garage with an 18 wide door that was 8 high. Good Lord I miss all that storage Now I live in a neighborhood with an “oversized” garage. Oversized my big ole butt! Yes it has an 18 wide door, but backing up the hill I have to drop the motor or indeed with the breakover I will take the top of the motor off with the 7 high door. On a positive note this is the main reason I started looking for a shorter boat and ended up with my Sabre FTD If I were to build another barn like you are I would do it the same way I did before, LOVED IT!!! If I had to back in I would indeed oversize the doors.