Price sounds good and the combo is great, that said and as was alluded to; "super clean" means vastly different things to different people. I would HIGHLY recommend you find someone local who is knowledgeable in Ebodies to look the car over before dropping the cash. This site has many stories of people buying their first Mopar muscle car and ending up with a a very shiny project car they cannot drive and use. There are some significant issues that could lay just below the surface that could end up costing you a lot of money and worse completely souring your passion for this hobby.
If you dont know someone local, just ask her and there is bound be a member in your area that will help. Worst case, I am sure we would all be happy to help develop a check list for you so you are not completely blind.
I say all this because $20K for a 4 spd car is on the low side for sure meaning that if it were really "super clean" it should be higher is price which then begs the question as to why it isnt. In this day in age, everyone has the internet and every seller has a sense of what their car is worth (usually far more than is real). Not saying it isnt legit and a smoking deal but ... If on the other hand you meaning of super clean is that it is all there, needs a fair bit of work, doesnt have a lot or any rust, then disregard what I am saying.
If on the other hand it is very shiny, it does run and drive, interior is intact etc. I would be very cautious because a car in that condition would usually start much higher. $20K isn't an insignificant amount of money so its worth going in with both eyes open. Not saying it isnt a good deal, it very could be, but then again it could be a hand grenade with the pin pulled.... and $20K could turn into a lot more quickly.