Jon,
Welcome. I too own a 356SC with all matching numbers. With regards to the W113's, I could be all wrong here, but I have gotten the impression that the value of a Pagoda is not affected as much by whether or not the engine is original. It's not like Corvettes or Porsches where "matching numbers" is critical to value.
I would love to see what our members think about this topic. With our Pagodas, for me, an un-hit, un-rusted, straight body, with matching body panel numbers, is more critical. If you have a great car like that, with a replacement motor, I honestly don't think the value would be hurt that much. That being said, a great car, WITH the original engine, would be worth a bit more.
John Mancini
Chatham, MA