since you have a 48" floor, you should have no stability problems with a front deck. I would only make the deck for the portion infront of the middle bench up to the front deck. Make it even with the middle bench top, and level it up to the front of the boat. You may have a little lip at the front, but that won't be a problem. You can glue carpet directly to the aluminum bench and front deck, so that shouldn't pose any problems as well.
As far as carpentry books... well, I'm not real sure. But this site is only a click away to find out what you want to know about. For starters, decide how much money you want to put into the mods. Aluminum angle will be lighter in weight, as opposed to using wood support, but it's more pricey. Since you are 250lbs, I wouldn't suggest any smaller than 3/4" exterior plywood (non treated, the chemicals in treated wood will eat up your aluminum). For bracing, you should be alright with supports starting at the front of the bench seat (3/4" shorter than where your deck will meet the top of the bench), and then more bracing running to the front of the deck.
If I get a chance tonight, I'll draw up a diagram of your boat showing where supports should go, as well as making storage space with accessible lids. Once you make your first cut, you'll have more of an idea of what to look for and what needs to be cut at what angle and length. If you can find a refrigerator box, you can use that as a template for your deck, and then transfer that to your plywood for cutting.