The motor, under heat will suffer from the alloy pistons, and valve guides being "cold". The valve stems, and pistons will be hot, and more-so the first few seconds after you get on the gas. The heat expands the moving parts, decreasing their clearance, and doing damage to the engine. Not a good thing.
there are 2 common ways around the problem, the first is to just build the motor sloppy, and intend it to run ice cold. Easy to do, most common, but it brings a couple of problems with it... engines are more efficient at a warm temp, and oil flows far nicer warm. There is far more of a risk of washing a bearing out in a cold motor (Love that squealing noise!). Although common in muscle-jets, it's not the greatest system to use. Even built-loose engines will suffer from cylinder flaring at the bottom from running too cold. The top of the jug gets hotter from flame contact and runs tighter, and the piston runs true in the bore, the freezing-ice-cold bottom allows the piston to wobble a little with the action of the connecting rod at BDC, the off-square piston causes additional wear to the jug and top ring. Engines with short piston skirts are really affected by the loose piston effect. (read any long stroked motor, like the 383 & 6.0 vortec)
The better, far more professional way to handle the problem is to install a thermostat and allow a bit of warm-up time before you start running the engine hard. The advantages are found in a more efficient motor, cleaner combustion, FAR better seal of the head with a conventional fiber/steel gasket (the head and block expand to a tighter fit) sensors for electronic systems will operate properly... the list goes on. With an alloy head, there is no question, if you don't like warps, you need a thermostat. It used to be hard to find the 4-ports, they are a bit of a relic from long ago. When they are closed they bypass almost all of the incoming cooling water into the overboard dump, the water is still flowing preventing damage to the pump, and the thermostat allows air to escape and the engine fill properly.
Most any commercial built automotive long block will function perfect in a marine app with a 4-port on it.
With a non-thermostat setup in a jet you have a ball-valve to close off some of the water flow, it's a nightmare, and you are always jacking on it. Unlike a conventional water pump, a jet produces ALLOT of variation in flow, and pressure. where you can "warm-up" a regular inboard somewhat easily, you never get a jet to be above 100 at idle, and not be lake-temp at full throttle. Think of the amount of water that comes out of a 3/4" pipe nipple at 100 PSI. That is going to take some heat to warm up! Now that flow at idle will be a trickle. Enough to cool the engine, but just. With a I/O the pressure is going to vary from 4psi to about 10 psi, More or less, that is going to be far easier to set temp with just a ball valve.
The second problem comes from any engine's #1 killer.... Hot oil, or better, way-too-hot oil. With no oil cooler a big motor will heat 9 quarts to 320-330 degrees in 15 minutes. That is bad, really really bad. Installing an oil cooler fixes it perfectly, and it's a must-do... but (always has to be a but) if you are regulating the motor temp by cutting flow on a ball valve, you are also reducing flow through the oil cooler, and it becomes nothing more than so much leaky plumbing. By using a 4-port the water still flows through the cooler at maximum rate... a good thing. Even a cheap cooler will drop scary-hot oil to a nice 220 deg in a matter of a few minutes of slow cruise. (keep your oil above water boil-off temp for the sake of corrosion elimination).
You want a thermostat... you REALLY want a thermostat The loosey-goosey engines are perfect for the extreme end of performance, but they are, like any high performance mod, a big trade-off in reliability and ease of use. Allot depends on your chosen application.
Man, that flex plate wobbles allot. I have never had one shear, but the flex while the first piston comes to compression is amazing. You are dead on, the added shimming keeps the run-out from trapping the pinion gear.
That 454 is a common beast, I have seen more than a few of them in boats, never knew anyone to use premium in them. It is right on the verge of needing it compression wise. It needs to run plugs 2-ranges under stock. If I remember right stock is an R43TS. I use an R40TS in my GMPP 572. The fact your marine distributor doesn't have vacuum advance, and the engine will be running 160-170 rather than 210 degrees will cure most any predetonation problems. (Another reason to like a carb, it's easy to just jet a little richer and cool off the combustion temp)
The difference between the 502 and the 454 is easy, the numbers look cooler on a sticker on your hull!!!! (The testosterone effect
) For our purpouse it's a mute difference. 48cid isn't that much displacement.