Have you ever had it on plane?
a single 5.8 in a 2755? That engine is going to have to be making 100% of it's rated power in that boat. Start with a compression test of the engine and look for any differences in spark plug appearance. Clean flame arrester? Good gas?
IF compression good, Check advance weights and springs underneath the points plate in the distributor. Rusted stuck parts may prevent timing from advancing.
Check fuel system and fuel pressure to the carb inlet when attempting plane and compare to specs.
Any water in the hull or excess weight in that boat is going to be an issue too. Check how it sits in the water compared to other 2755's.
Then there's a few other things to look for like exhaust restriction (flappers blocking exhaust) or failing/clogged manifolds.
Prop pitch.
But worry about none of that until you are convinced the engine is in olympic condition.
That's what I hate about under powered boats. You can't get on plane when even the slightest thing is amiss.
Ever try to start off in a standard transmission car in 3rd gear instead of 1st gear? That shudder is what an engine will do when it is asked to deliver more torque than it can physically produce.