What would be the expected results of taling
a basic 454/330 8.5:1 flat top pistons (stock) motor with improved breathing - intake manifold, heads (781's with 2.25/1.88 valves), good exhaust stock cam, running well
installing a good used set of 1.8 roller rockers, again changing nothing else.
Really interested to hear, anybody else here try this?

Stock sucks.

So do jet engines but neither have 1.8:1 roller rockers

Why even bother? I don't believe you will gain anything but a headache.

at least you can use the heads and intake on the new motor

Sell the roller rockers and use the 150 bucks for fuel.

The first half of the title of original post, is the answer

20HP, maybe..

Seriously, there are a lot of decent running reliable older 454/330 out there, everybody says to change the cam, i was wondering if this may be a cheap easy way of trying to improve on what everybody knows is a major restriction to these.
Not everybody knows how or wants to or can even pull motor and change cam. Anybody can change rockers.

About .020 more lift and that's about it. I don't think you will see any performance.