That white stuff was a PIA. Get yourself a nice quality chisel...preferably a concrete/brick splitting chisel and a hammer. Get in between the white stuff/putty and whatever surface it's adhering to and give it a few whacks. That putty SC used is very solid, very dense but also very brittle.
Lookin good Yamt125. The older SC didn't have the same arrangment in the hull as the later SC's. My 1978 doesn't have stringers like you, instead it has these "pontoon" type stringers filled with foam. For your project I'd take a look at Jason "Trayders" project and make it my template.
http://www.classicseacraft.com/Trayder/index.html
The MA I think is more comfortable and utilitarian, but the SF is a far better looking boat...much nicer lines!