I haven't done this yet, but am planning the same modification on an HB. My plan is to simply weld up the old hole in the firewall, drill out the new location as close to the frame rail as possible, and use steering shaft U-joints (very common from any race car chassis supplier) with an intermediate shaft (possibly supported by a bearing if necessary (again, check out the race car suppliers) to reconnect everything. Similar set up to this:
http://www.cadvision.com/blanchas/54pon ... 23str2.jpg