This is how we dream of crossing America here in the U.K.

Hello all, first post here. I have been wanting to do the cross country road trip for a while, and now that I'll be finished with college, I'm thinking of doing this in the spring, and in my 68 Chevy Impala. I want some input on how smart of an idea it would be to do this in a classic car.
How cool is that ! Welcome to the R.T.A. forums.

Also could be great for some photo opportunities on the way. I of course will be getting it fully serviced soon as it's been sitting in my garage for a while,
Speaking of photo opp's, have you got any you could post up of the "bruiser", I love these classiics.
If it's been sitting for a while then I would [as you suggested] get it looked at fully and serviced. Although I would personally take it for a couple of "shake down" trips closer to home before embarking on the big one, when cars have been sitting they can throw up the odd surprise in the first few miles, [normally little things that are disruptive.]

it's got a tricky manual transmission.
Goody, A proper car ;-)

Anyway, is this a really bad idea,
Not at all, it could be a great adventure and one I would love to read about on your return. However I think the decision will be easier to make when the old girls had her "health check" and you have an informed opinion on her condition.

Have a look around the forums for ideas and if you need any help with your itinerary then ask away.