Too much? Wont fit? Too big?

Yeah that covers most of it. The way I see it even if you just go small block you already need a new radiator and exhaust, you need conversion mounts or a new K frame, you need a different transmission, and even with a small block you should ditch the 7.25" rear axle which means a new driveshaft too. /6 torsion bars are lousy even with /6's, so count those in either way. What's harder about the 440? The headers will cost more and you only have like 2 choices. The schumacher headers will fit with stock power steering still. You need to upgrade the rear axle for sure with the 440 swap, but really that 7.25 isn't going to be a lot of fun behind a small block anyway. It will work, but if you have too much fun you'll be picking up the pieces and getting towed home.

You can also look at Schumacher, they make the conversion engine mounts so you can keep your /6 K frame either way
/6 to small block
'67-72 A-Body S6 to SB Installation
/6 to big block
'67-72 A-Body B/RB Installation

As for the 440 transmission it's probably just a 727, unless it was out of a motorhome there's a good chance it's the same as the rest of them. I'm guessing if it was a manual he would have said so, but maybe I'm wrong.