I think you can find a decent 70 for $7k, but you will have to look hard and almost certainly have to travel for it, so I guess travel and trailer costs would bring the price up. So, yes, in the end it wont be easy to keep that budget.
I still say keep looking for that 70 that you really want though.
Think about it. You will be spending years on it, and it will be a part of your life, so you need THE car that you WANT. Dont settle.
Besides. Do you really want to spend the same amount of time on labor, and money on parts, to end up with a car worth maybe HALF AS MUCH?
72-74s are great cars too, but will probably never be worth nearly as much as 70-71s.
I feel bad for the red headed 72s. Just because they never came with hemi's or big power, people piss on em. It's not right, but thats the way it is.