If the panel is on a basement wall you may have enough wall width to use a sweep (your Grainger item). It would be MUCH easier. Carefully plan. You may be able to shorten the sweep. If you cut one side back you may get to where the PVC is not round anymore.
Back-to-back LBs, with the back of one LB running into the back of the other LB, are a GIANT pain to get the wires into. Maybe a BFH and one of them alleged dynamite torpedos.
For one LB and your short dimensions you can put a 90 degree bend in a wire and push the wire in both ways at once. Do one wire, then the second, then the third.
LB ("conduit body") size is regulated by the NEC in 314.16 and 314.28. They are large because they need to be to get the wires into them. Restrictions kick in when wires are #4 or larger.