For the keel resting on the cross member, are you going to raise the bunk height to get it off there or just plan on using the roller to push it off?
I know you said the the 4x4 made it sit to high, but the extra 3 1/2 inches would make it so you didn't need a roller.
As for the Rivets, I when through 2 (Harbor Freight and Amazon special) before I finally spent the money for a true rivet gun (Used Airplane rivet gun). Lets just say it made a world of difference. I had to turn down the PSI to under 40 so as not to damage the hull/rivets. When we got it dialed in, Admiral and I could do a single rivet with a single 1/2 second to second burst depending on thickness. Bucking bar also makes a difference, started with a small 2 lb and went to a 10 lb. Did we do it right, probably not, but it worked for us.
SHSU