Cling's has the adapter.
https://www.clingsaftermarket.com/addtocart.asp?prod=43
Fairly easy job to do.
There are different gear sets and interchangable parts combinations. The later synchros will fit the earlier gearsets and cases, etc. VanPelt Sales website has lots of good info.
http://www.vanpeltsales.com/FH_web/f...sID_3speed.htm
I like the 37-38 gearset which has a little higher ratio than the Model A. My engine is not stock and I use a 3.54 rear end. Very nice for touring. Others will have opinions.