I have never seen a billet rear cap, other than on an aftermarket Genesis block, but I had billet caps put on my 427 center oiler block, and that included the number 1 cap. It was billet, but just a 2 bolt cap. They were not Pro-Gram caps, but I understand they have them available, but like others said, if they cost that much, just use the stock number 1 and 5 and retrofit the 2, 3, and 4 cross bolted caps.
It will end up costing a grand or so, 350 for the caps and then the rest for machining, fitting, and the line bore. It is pretty labor intensive and you better have a good machinist that is a perfectionist, and has done it before.