It's a 0, not a 6.
Very very very common 4" bore block from the 70s, maybe THE most common one. Came in cars, trucks, vans, boats, and everything else. Was used in everything from 69 Z28 302s to phone company vans, and everything in between.
Typical smogger heads. They came on the mighty 165 HP 350s of the mid 70s among other things. Nothing with over 185 HP though that I can recall. A126 = the 12th day of the Ath (1st, Jan) month, in a year ending in 6 (probably 76). That might be the most common SBC head casting in existence.
So you've got a bunch of very usual SBC stuff. Nothing out of hte ordinary in any way.
Impossible to guess how much power you're making, without knowing alot more about other things (pistons, cam, head work, exhaust, etc.). However, most likely, not more than 240 HP.
No way to know what any of it came from. Well, I take that back: it came from a GM product. No way to know whether it was a car or truck, what kind of car, whether any of it was underneath a 2- or a 4-barrel carb, etc. Not that any of that makes the slightest difference now.
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Numquam ponenda est pluralitas sine necessitate.
— William of Ockham, from Quaestiones et decisiones in quattuor libros Sententiarum Petri Lombardi
Roughly paraphrased into modern English, and applied to figuring out what's wrong with your car:
The simplest explanation that fits all the facts is probably the right one. | 
