289 was 9.3:1. ('68 FoMoCo Shop Manual)
As for "how much more Hp.." wrong question. As most "Hp" is measured at the peak of the curve, its generally not used and mostly for bartop racing and bragging rights (how often to you run your engine at 6000+/-ish RPM?).
And are you running a auto or manual? Makes a difference in how it gets to the road and how the change will be felt.
IMO more important would be looking at how it impacts the torque curve across the 2000-4500 rpm range, where you mostly feel the effect of the change (as thats where most folks do the majority of the driving, unless you run fast on the freeway with small tires and typical low ratio rear, typically 2.79 as noted above). Your cam and heads will probably be a limiting factor and a stock 289 c code is not going to run at 5000+ rpm real well, too restrictive to gain the full benefit of a 4bbl and good intake.
It will improve somewhat, but without complimentary head work its a limited gain.
Find someone with a desktop dyno and run some numbers.
If you are running the typical C code single exhaust, swapping it to duals would also help a bit.