225 slant 6 or 360 v8

That slant will need some SERIOUS head work to be at 300 HP, and I seriously question that is there. Is there a for-real dyno sheet with it or have you seen it run up the track? And if it is, it will likely be pretty peaky and not likely a good drive-around engine. All that changes if it were turbo'd but at 10:1 CR it won't be. (Some of the leading edge rally cars are turbo'd hard with 10:1 CR but that is not this....) I would be very suspicious of the seller at this point unless..... there is one guy in the Waynesboro PA area who could do an engine like this; HE I would listen to. What part of PA are you in?

All the above is well said.... despite my strong liking of /6's (I have owned 4), if 14's is what you want, the 318/340/360 is the route to go. I don't often say that....