There are numerous aftermarket vendors that can provide wheels to your specifications. Wheelsmith, Vintique, and Summit to name a few. Aftermarket wheels don't accept the stock hub caps, however hub caps can be modified to fit. GM used the 6 lug bolt pattern from the 30's and into the early 70's. There were some differences in width, style, and hub cap mounting. Chevy TF trucks (55-2 through 59) used the same basic wheel as your GMC with the same hub cap clips. They may be more readily available.
Fred
