Yeah, there are usually several of these later parts to convert the 72-up TH400's to the 34 element sprag, for sale on Ebay.
http://www.ebay.com/sch/i.html?_odkw...&_sacat=171114
There are several brands of 36 element sprag / drum / race units available for the TH350. Here's some on Ebay, including one that says it has a "mechanical diode" instead of a sprag or roller clutch. Anybody using this kind, or have any info about 'em ?
http://www.ebay.com/sch/i.html?_odkw...&_sacat=171114