My information comes from several sources I've accumulated over the years
I have a hand typed spread sheet I did in the 70's from some GM sources I had access to, a copy of several lists I've been given over the years by fellow 'numbers-guys', a copy of the 1974 Parts Master Cat from GM with all the decode pages in the front, 10~15 different published reference sources from various authors ... you get the idea
I never use one source to quote a given decode, I refer to several and cross-reference the information to see if it makes any sense compared to the unit I'm trying to decode.
Some sources are just dead-wrong, and then they get 're-quoted' by several other sources until they become "fact"
Specially see alot of that in the last 10 years or so with the proliferation of data sites on the net. ...
As far as 'installing a 4-barrel and what would it boost power output to' - that's really hard to answer.
Is the engine just the way it left te factory, or did someone rebuild it before installing in you vehicle? It would be a 'normal' procedure to freshen up a used engine during a transplant - even if the owner just went to the nearest "Jiffy Engine" place and bought a replacement assembly.
If so, chances are they used fairly ctock parts and the present output is somewhere near the original factory rating - maybe ...
If you would like to start some improvements in performance I would suggest you start with the exhaust system. A mild set of headers and some reasonably free flowing pipes should net you some noticable 'seat-of-the-pants' improvement and get you should be set for more HP upstream.
Then I'd check the cam and install a mild 268~272 range cam and a dual-plane aftermarket intake manifold to match your selected 4-barrel carb.
All the above should serve to wake-up your present engine for a reasonable price. A HP output estimate would probably be in the @275~<300 range - depending on choices.
From there, the sky is the limit
You'll need to seriuosly consider where you want to go next - if you wanted more HP after that, you'd need to consider the bottom-end and heads in the equation - and things start getting expensive quickly at that point
Hope this helps some, and others chime in;