First off, welcome aboard!
I days away from buying a numbers matching 1973 barracuda with a 340 automatic. Everything is original. The frame is strait and never wrecked. Here is the things I would like knowledgeable opinions on. The person I'm buying from does rotisserie restorations and he will be doing one for me.
Is it better to have numbers matching or put a 440 into it? Collector value wise
Collector value wise, the '72-'74 cars don't even come close to their '70 and '71 sisters. Even being a numbers matching 340 car. If you're looking for an investment you'd be better off looking for one built in the first two years.
At this point, build it the way you want to. Put the 340 on a stand and if you need to, you'd be able to sell the original engine with the car.
Also how bad is it to change the interior to black from the current white?
Not bad at all. Seat covers, door panels, quarter covers, kick panels. All are available. Or if the original plastic is in good shape you can send it out and get 'em covered.
But, this comes back to your original question. If you're really concerned about originality you'd keep the interior white.
It is currently the brown color from the factory - I would greatly prefer painting it Plum Crazy or red. Will the color change hurt its desirability.
Like I said, this car is no where near as desirable as the first two years of production. A color change on a '73 might just be more appealing to the next guy.
What modifications should I consider doing to the engine - if any. and in what priority assuming I can't afford to do everything.
Which engine? The factory 340 or the 440 you want to put in it?
Look, a couple of things you need to figure out. What's the car
you want? Having someone else restore a '73 - even numbers matching - is gonna be a break even prospect, even if the car doesn't need a lot a work and the car is sold for top dollar. If you're looking to put money into it and flip it, find something more desirable that's still screwed together, say a '70 or '71 340 car, or a Six Pak car, or a Hemi. Top dollar, yes, but top dollar when done. This car just isn't going to bring the money.
That being said, if you're worried about the whole numbers matching thing, you need to go back to 100% original. Engine, color, interior, just the way to car rolled out of the factory.
But it seems to me, from reading your post, you found a car that you'd like to build your way and you can't get past the idea that the original 340 is in it and you're afraid of screwing up an original car.
Build the car you want. Black interior, Plum Crazy (overused in my opinion), black interior, 440. Or leave the original engine in place and step up the performance. Bring up the compression, good cam, head work, headers, and let that little 340 rip.