What did I buy?!

you did pretty good for a running 360. i bought a moderate 318 a few years back for $650, got me into the low 14's and i think i resold that engine for a grand.
I will say remember to be realistic about the never ending quest for the 30mpg daily driver that runs 12's.
350 (?) horse is going to struggle to see 12's without a hell of a converter and gear set...both of which make it less streetable. plus being sold to you as a "torque" towing engine, could be his way of telling you it's out of RPM at 4500. not really going to light up the quarter mile with a comp 268 they rate to 5100 RPM.

you'll just gave to get it in the car and see how it acts/revs unless you are doing a tear down. good luck and good find!