Been there, done that. Weld up the cracks as best as you can. Then use some 2x2x.250 angle iron all the way across on every rib. Stitch weld it with mig. The wire feed welds over dirty old aluminum better than tig. Plus it's gonna be alot of welding.
36" from the deck to the top of rail.
Yea, That boat has definately come a long way. I still remember sweating my *** off welding on it in the summer heat.
Here are some pics of some rails I put on this bad boy of an air boat this weekend. Customer wanted angle iron mid rail to attach his lights, made this job a lil trickier than most.