I agree with everyone. Stash it in the corner so if one day you want to sell it, you can include it with the sale. If you plan on just keeping the car and enjoying it, the #'s matching thing isn't going to do much for you any way in terms of value of the car since you aren't selling it. Not to be a downer
but a #'s matching '73 Challenger with a 318 isn't going to fetch you much more $$$ than a non-#'s matching '73 Challenger with a 318, IMO. A little more, but not much more.
Like 71chally416, go with a 360. It'll bolt in with out mods (as opposed to a big block) and are easier to find that a 340 (and costs much less). You can find 318s pretty easily for low cost but while you're at it might as well increase the cubes to have some more fun. Costs will be about the same to do the mods to a 318 vs. 360 but you'll have more cubes on your side.
Have fun!