This may be a lot more work than you need/want to do. If the doghouse has never been off the car, I would leave it on as the factory installed it. Your cracks can probably be repaired correctly from the top side. Usually, only serious body damage needs to be repaired from both sides and at that point you are better off replacing the panel instead of attempting to repair it.
If you feel the doghouse has to come off and if you want to do the job yourself, my advice is to purchase two or three of the Corvette restoration books and read how others have done it to familiarize yourself with what is involved.
Purchase the assembly instruction manual for '81. The AIM is not a How-To reference, but it shows how the cars were put together on the assembly line.
Purchase a couple of the Corvette fiberglass repair books and read through those. I prefer the Glas-Ra series. Eckler's book is also useful.
Once you have done your homework and understand most of what will be involved with remving and rehanging the doghouse, then you can decide whether to proceed.
