You are correct that the biggest engine you could get in a base Challenger was a 383. That said, you post is a little confusing, you said it was originally a 318 car and then said you asked if it was originally a BB car..
As to value, well that is highly subjective as I am sure you know. Its worth what someone will pay, however for me personally I think its high but thats without actually seeing it. If it is an originally 318 car but exceptionally well done and you never intend on selling it, then it might be good. To that point, regardless of how it started life, if it isnt in good shape structurally, electrically, etc. then the price is too high either way.
Perhaps another way to look at it is this; if you were going to build a car starting with a mediocre donor (meaning not a ton of run, good structurally, not hit, etc) you would be hard pressed to build it for what he is asking unless you are capable of doing all or the vast majority of the work yourself. So, what I do is begin looking at the math (read dollars) as to what I don't have to do to the car in question. I start with a list of the things needed to arrive at the vehicle I want (engine, trans, paint, interior, electrical, suspension) and give values to it all. Then I look at the car I am considering and deduct what I don't have to do to it. In the end it is all reduced to dollar figures. For example, if the paint is good, that 5-10K off what you would have to spend. Since you are doing a TA clone, the drive train doesn't matter (at least the engine), but you might be able to sell it. Look at the suspension, interior, etc.
Lastly you need to determine what you intend to do with the car; if you think you might drive it for a while then sell and move on, this probably isnt the car for you as I don't think its going to climb in value dramatically as a clone or tribute and if you have to dump another 10K into it to get where you want to be; you are in it for 54K.
Its probably not a horrible deal at 45k, definitely not a steal either, but if you have to spend a lot of $ to get the car in your head you might want to keep looking.. just my