[Matt Monson]

Here's a data point for you:

http://forums.pelicanparts.com/porsche-914-used-parts-sale-wanted/626518-1971-914-w-350-chevy-los-angeles.html#post6217087

Though a lot of it depends on the condition of your starting point vehicle. If it's really solid cosmetically and mechanically, the swap won't add a lot of value, but won't hurt it at all.