Is it that easy to jump to a gen 6 setup? Just the cam and timing chain? So I would need to get the cam retainer plate too, and would I need the roller tappet guide if I order the cam kit that comes with lifters? Or would that even work on a Gen 4 block?

On Mark IV of Gen V blocks, there is no provision to use the factory roller system. You would have to use link bar style rollers. Gen VI blocks also have a bolt on plate to keep the cam aligned, as there's no forward thrust from the FT beveled lobe.

The Merc 420 has a lot of decent parts in it to start out with. It just doesn't have a lot of compression, and the piston to deck clearance is a little wide to just cut the heads to reduce chamber cc.

On a lightweight boat, the 741 will work well with your other components but you'll be leaving power on the table with your low compression.

On a heavy hull, the 741 will be a little soft on low rpm torque with a 454.

Optimally, if you could snap your fingers, you would zero deck the block (piston to deck height of zero), you would bore notch the cylinders to unshroud the valves, and you would drop in the 741 cam and springs, along with tie bar roller lifters. Your WEIAND Intake is a decent piece, and your carb isnt really holding you back. Ignition is also adequate.

A lot of guys pop in a hot cam and expect another 500 rpm on top end due to the added power, but find that they've lost enough bottom end to make it sluggish to plane with a full load.

Gen V blocks can be drilled and tapped to bolt up thre Gen VI cam plat on the fron of the block. Way better solution in my opinion...than a cam button.

OP would likely get better results by spending similar cash going with a set of aftermarket heads.

Gen VI block, is a different story.

All the blocks I’ve seen already have the bosses drilled. I believe they used them with gear drives in the heavy duty market.

gen5 I took apart yesterday. Came out of my 94 top gun.