Expected HP increase?
 

97 donzi zx25 w carbed 454 mag. All stock. Adding Keith Eickert exhaust. What kind of HP bump should I expect? Also considering adding an air gap intake...how much more will that yield.

Also looking for prop advice...now it runs 63 (speedo, not gps) at 4800 rpm with a 21p 3blade mirage plus.
It wont take but a pony or two more to hit the rev limiter...what prop should I ilook at after the exhaust and intake mods.

I'm afraid to say this but I doubt you'll see any gain at all. It takes about 20 hp to gain 1 MPH and if your lucky thats about all you'll gain from what your considering. Look into a bigger cam, roller if you can swing it along with roller rockers. With the mods you're thinking about, your all set up for a cam but without that your gains are going to be minimal at best. As for the prop, nothing is going to change so no need to change that. If you do add a cam, your engine will most likely spin higher, so even then your current prop will most likely work. It would take a BIG power gain to need a bigger prop!

Ditto on little to nothing gain in speed just bling factor!

A cam, carb, intake and exhaust upgrade done with the proper combination of parts will get you 600-100hp.

Exhaust alone might get you 20hp or 1 mph. An intake without a cam and carb change isn't really worth anything. The magnum intakes are pretty decent.

You've got 350rpms before you are hitting the rev limiter. That is about 50-60hp.