Your VW setup may work for testing, but there is the 180mm clutch vs. 200mm clutch to worry about, and 6v. vs. 12v starter teeth, etc. If you can get the engine number, check it at
www.356a.com in the lookup there to see what kind of engine and what year of production. If the engine isn't in the db yet as being "known", please add it (location/year and the fact it is for sale) - many 356ers are looking for the original engine case for resto/concours purposes.
For putting it in the KG, aside from physically getting it in there, the 12v vs 6v and the clutch size will be the things to worry about. I think there is an article or two on
www.356registry.org that detail putting a VW engine into a 356, that may give you a head start.
Edit - you'll want a copy of Elfrinks Technical Manual for Porsche specific data (valve clearences, torque values, etc) but when working on my 356 I spend a lot of time with John Muir's "idiots guide to keeping a VW alive forever" book, mostly because I'm a mechanical idiot.