Thanks for the continued discussion. However, I am sort of stuck lacking available resources and expertise.
1. Finding an oversize seat post is pretty much a non-starter as they aren't available in the size that would be needed to compensate for the oversize bore.
2. The existing aluminum shim is only 4" long so is a long way from supporting the length of the post. I might consider replacing the existing shim and starting over... if I could find one. I suspect the builder knows of a source for these but I don't. Having a machinist make one from scratch would likely run into hundreds of dollars.
3. I could buy and figure out how to use a reamer but it is currently pointless because I don't have a replacement shim.
4. All of the local bike shops are pretty much parts changers on big name mass market bikes. They don't deal with customs or frame specific issues. They just replace frames as needed. They don't have the expertise and proper tooling to replace a seat post shim.
5. As for the tapered bore I can pretty much guarantee that whomever reamed the shim didn't know what they were doing and likely added too much side force on the handle when reaming. The builder said that they don't cut the slot in the shim until after reaming and that this could cause the bore to open up slightly when the slot is cut. I am afraid that I don't buy that story. Assuming that the aluminum shim is press fit into the tube I would expect the bore to close up rather than open up. It also tends to show that the builder used a straight flute reamer on the shim. BTW, The bike has not yet been ridden so that couldn't have caused any of the excess play.
6. As for finding a local machinist my part of the country is not known for manufacturing. What little manufacturing that was here has mostly disappeared over the past 3 decades.
7. In case someone might know of resources that I don't I live in central NC near Raleigh.
Thanks,
Steve