We just ran into the same thing, we wanted TRW L2323F30 pistons for my Kid's 455 Olds. Found out they are on National backorder, and it could be a year! Then I thought maybe we could find them in 40 over instead of 30 over and I made a few calls. Turned out Jegs had some of those in stock, so we took the engine back and had 10 thousandths more taken out of the bore. He ended up with a few more cubes, and we didn't have to wait another year.

Maybe you can find some in another size. As mentioned it sounds like these are 40 over already, so maybe you could go to 60 if your particular block can take it. I know that kills most chances of a rebore down the road, but it might solve your immediate problem.

Don