Electrician came yesterday and here is what he did. They stuck to their guns that the plug etc. had to be multiplied by 1.25 requiring a 60 amp plug for a heater using 42 amp. Since I am building for the future, including the possibility of someone using this plug for something else, I didn't argue about paying for more expensive plug and receptacle. The good news about the 60 amp plug is that it is easy for me to attach and detach from the wall outlet; it can be grasped with two hands with no danger of getting a finger near the prongs. I liked the idea of the Twist locks better, but they had already bought these, and they will work ok.
So he pulled two new 6 gauge wires and a new 8 for a ground from the panel next door, into the box and then on into a smaller conduit over the door, and ended in a more convenient location to hang up the cord and park the heater. A 50 ft 6 gauge SO cord was attached to a 60 amp plug at one end, and the other end directly to the heater via a strain relief gadget as described by Torch. I had said 35 feet of cord would do, but the electrician pointed out I was going to pay for 50, may as well use it. My thought was you can always shorten it, but you can't make it longer.