UPDATE for anyone still around:
Today I went in the crawlspace and the old addition cinder block foundation runs under the cantilever (I have no idea how deep those cinderblocks go) . On one end the plywood that covers the bottom of the cantilever pretty snugly, but the other end has a gap that runs from 1 1/2 inches at the base and 1/2 inch at the end. I was thinking of filling that gap in with cement.
I dug a 4 ft level into the ground until it sat level and measured the distance from the top of the level to the plywood that covers the bottom of the cantilever beams. I measured it at the base , 1ft from it, 2 ft from it, 3 ft from it & at the end (41 inches from the wall). I did that for most of the joists. At the worst there was an inch gap, but most of the beams showed about a 3/4 in gap. Turns out I also used 2x8's, not 2x6's, for those piers I put under there last year.
QUES: About sistering in 2x8's (about 10 ft long) in the basement. I could just try and hammer it in as deep as I can get it, but I don't want to force the floor up above and risk unleveling the countertop etc. But since I have an idea of the sag for those 41 inches past the end of the basement foundation, should I cut off a small triangular region from the top of each beam based on my measurements so I could get the joist all the way to the back of the cantilever?