Dave, for starters, Chilton's is quite good. In fact, that's where I got the specs all in one place, and didn't have to search the big bulky volumes for them.
It depends on what kind of work you are doing to determine which books you need. Are you going to trick it up with after-market or later production modifications? Then interchange manuals might be a better bet. Others on this forum would have a better opinion on that, I think, because I am leaving mine mostly stock. The biggest changes I'll do to mine is add power steering, add front disc brakes, and change the 352 to a 390. For the major modifications like the brakes and power steering changes, I'll use the knowledge of the great folks on this board. Other stuff, it depends, and what I have access to I'll share as I can.
That doesn't mean that the original manuals won't be of some value, but $75 is a bit much, in my opinion. For example, I got the 1968 (and earlier) Ford Truck Service Bay Parts books (well-used, of course) for $50 on E-bay a while back, and got the original (yes, original) Service Manuals for F-Series for $35, all four or five volumes of it.
The deals are out there, just keep looking.
Good luck!