Hi Drey, welcome to the site. A good rule of thumb is to look at the Max RPM range of the engine, then compare that to the max. RPM when the engine is at wide open throttle (wot). If the rpm at wot is higher than the max rpm range of the engine, you're under propped. If your max rpm at wot is under the max. rpm range for the engine you're overpropped.
The specs I found for a 1992 Mercruiser 4.3L was a Max. RPM Range: 4400 - 4800
What is your max rpm at wot when this behavior occurs?
Also take a look at the prop calculater at mercruiser: