I have gotten some from these guys. They show a 16x4.5. They are familiar with Bonneville needs and can weld 360* etc.: http://www.earlywheel.com/index.html
it took me almost two years to get my very plain and basic steel wheels from jason at early wheel.
Clearly becomes a "your results may vary" deal
First set I got, had them in about a week including shipping from CA to MD. Indeed, last year I wanted a set, they didn't have the particular rim size in stock and I didn't have time to wait on them getting it in and went elsewhere. We had dealt with Stockton also and had same no delivery for months results.
For my purposes, stock type plain wheel also worked and I got them through Summit. Don't know about Jim's 300+ but there are these:
http://www.summitracing.com/parts/wvi-14-64212234/overview/ Get them shipped today. Weld them up 360* . When I built Jim's car originally, I got the wheels made locally using a VW rim and welded 1/4" plate into them for the offset. Udo Horn and I did basically the same thing for the front wheels on the Bonneville Bugeye /GMS car (see his build diary)> With the major offset Jim might need, doing something similar in a 16" size might be the way to go.
