Are there any options to improve the horsepower and torque for an original base 350 in a 68? I know there are aftermarket options but my intent is to keep the original block, cylinder heads, intake manifold and 2 barrel carburetor so it still appears stock outwardly.

If so what is the realistic gain I can achieve?

It has the original 2 speed automatic transmission.

nope
if anyone tells you otherwise they have no clue what the F they are talking about.

Strap a rocket to it's [censored] and let her rip!

I'll give you the unsarcastic answer... which is totally not me but.. i'm one of the few remaining "350 guys", or at least that's how it feels.

You can do great things with a 350. The 68 originally had #17 heads. A great 350 head. And you can probably go .060 overbore on the block. BUT... even with heads all worked and ported it won't make "power" without better flow in and better flow out. And no cam will change that.

But if you want it all "original", which it really won't be... throw in some better gears, add a dual exhaust, dump the original air cleaner. It will go ok but probably 15-20hp over stock and feel way better.

Now if you want to really build a 350 you can use those heads, overbore, swap to 4bbl intake and carb, pick from a variety of appropriate cams, add some long branch exhaust manis and a dual exhaust and easily get 325hp. If you have money and a dream you can make way more hp. Don't let them tell you otherwise. Yes it is way easier with a 400 or 455, and costs the same or less. Not the point. But with posi and some 3:23 gears... she will get up and go. Not a race car but plenty of getty up.

This a choice every classic car owner needs to make. Go original and love it for what it is... or just do it the way you want it and don't look back. There is no right answer but you can't always have both.