My SVT had similar symptoms when I bought it. It cranked and almost would start, but would not. I opened the cap on the overflow after cranking and it had pressure built up. I figured it was a blown head gasket pushing compression into the coolant system. I threw in a new set of plugs just to see if it would start before tearing it apart and it fired right up, but it only started for a hand full of times before the plugs were fouled. I tore it apart and the head gasket was blown, head was warped, I assume it overheated at some point.
Even though the plugs look good I would try a new set and see if it will fire up.