Terry, I would suggest not trying to make the pocketed joists work as fillers in your 8x8 girder. The compressive strength perpendicular to the grain is half of the strength parallel to the grain, the modulus of elasticity is lower perpendicular to the grain and it is going to shrink no matter how tight you wedge them now. Calculate the section properties of your girder at the joist pocket. I have repaired timbers with notches in the top using same- species blocks with the grain aligned with that of the timber. In this case only I count it as a full strength repair. (tight blocks with epoxy)
Better use T&G decking to distribute those wheel loads. Also I try to align framing/ sleepers under the wheel paths.
Good Luck, Chris Hoppe
