ED,
The timber bear blocks that I have seen were very heavy. You will need to trim a lot of metal away. The transfer passages may need to be enlarged also. Here is a link for the pro mac 610. It is a 60cc engine that uses a Walbro HDB 16mm carb.

Dave

http://www.acresinternet.com/cscc.ns...4?OpenDocument

http://www.acresinternet.com/cscc.ns...c?OpenDocument

Photos on ebay: http://cgi.ebay.com/McCULLOCH-610-TI...QQcmdZViewItem