well, this is pretty basic, bordering on the obvious....
If the cap is on top of the coil, then it's an HEI ignition, after about 1974. It's carbureted, so its before about 1985. If it's a pre-feedback carb (with a mixture control solenoid), then it's probably pre-'80.
That leaves the 4.3L (a chevy 350 with 2 cylinders lopped off) but I think all those had the distributor on the back of the engine ala' chebby. Could be a 2.8L, but I don't think any were built pre-feedback carbs, and I think most of those had externally mounted coils, when not DIS. The last choice would be the 229 and 231. The 229 was a chevy motor, distributor in the back I think. Front-mounted dizzy is typical of buick.
There were 2 major variations of the buick 231 in the 70's - even fire and odd-fire; I can't remember which, but the firing order is the key there. Chevy may have had their own variant of the 231 as well - the 231/3.8L was a popular mill all the way through the 90's and went through a TON of changes. (including the legendary Buick Grand National Turbo)
Hope I didn't insult your intelligence.