Virtually any General Motors steering wheel will have the same spline and taper. There's bound to be something that works- - - -maybe a Chevy passenger car wheel from the 1950's before they started going to the angled spokes? A dished wheel would probably defeat the purpose of shortening the worm gear shaft, IMHO.
Just checked- - - - -there are all sorts of original and aftermarket GM steering wheels on Ebay, in several diameters. Don't go too small, as the turning effort on a non-power steering system goes up as the diameter of the wheel goes down! Some of the wheels there are in the 14-16 inch range, which would be smaller than the pickup wheel, but still give you enough leverage to steer fairly well.
Jerry
