Johnny , they are called "core support fillers" by Ames, part no FP191J , sold by others too, Ames charges $99.00 for steel set , also sells fiberglass , but I doubt they hold well, you tend to lean on them...FiberGlass for $ 62.00 /pr (in my old parts book) part no FP191K

and yes ,called X fillers because of the stamping...
sometimes found on EBay...

you need in conjunction weth these also the "masticated" fillers, "seal kit" at the bottom ea side of radiator (closing off the openings at bottom..)check to see if there..FP454 std cars,or FP455K A/C cars 414.00 set...