From what Little I remember it was used in the small Stinson which pre-dated the 108 series. This was I think the HW75, and there is certainly at least one in the UK.
As for what to use it on? It is supposedly a direct replacement for the Continental O-200 (indeed the Franklin was originally called the O-200 and the Continental an O-205 - before the use of the Franklin died out in the US military).
The issues? Well spares are like rocking horse sh*t, the impressive sounding 125hp is like all Franklin claims - a complete fairy tale, it is more likely 100 to 105. Oh and finding anyone outside of the US who knows anything about it is nearly impossible.
I looked at them carefully whilst an outfit called Atlas Motors were trying to sell them in the US - they sold a handful but I think almost exclusively to exisitng Franklin owners and even then the engines were not complete.
Its an interesting thought but would I replace my O-200 with it? Afraid not!