As for how much hp you can make, here is a rule of thumb. This is just a rule of thumb, but it gives you an idea. Take the max intake flow of the cylinder, multiply by .25714 and multiply that by 8 (for the number of cylinders).
You can simplify that if you have a V8 by just taking the max intake flow and multiplying it by 2.057.
I googled a couple of flow figures for unmodified 2 bbl heads and found 186 and 192 cfm. Take those figures times 2.057 and you get 383 hp and 395 hp. That is a very general idea of what you could get with unmodified 2 bbl heads in a maxed out combination.
I keep saying "general idea" and "rule of thumb' over and over so I don't get flamed by anal retentive internet keyboard jockeys itching for a victim. Get back you fiends!
Because figuring horsepower potential is much more complicated than that.
Nonetheless the above formula is useful.
Take home message: 2 barrel heads could be good for a mild 351 in the 350-375 hp range. Some port work would worth the dough!
I think you can find flow figures for ported 2 bbl heads in the 225-230+ cfm range depending on the extent of the work. Maybe some Cleveland guys will chime in with some flow figures for mildly ported and fully ported 2 bbl heads???
225 cfm x 2.057 = 463 potential hp.
230 cfm x 2.057 = 473 potential hp.
That's not necessarily the hp you would get. It's an estimate of the max hp you could get if everything is "maxed out".
Any money you spend on good and appropriate port work is worth it.