I agree with Griff.....you can try it, but you may not be happy with the results of that cam.

If you really want to see a little more of a kick, then replace the stock Merc cam with a hydraulic roller cam that will peak the engine at 5400-5500rpm, a mild bowl/pocket port of the heads while paying special attention to the shortside radius. Depending on the cam you use and the exhaust system you have you may even get away with replacing the GM dual plane intake manifold with an open/single plane intake. (I think the stock Merc 420 came with a dual plane intake manifold).

Also, (if you can) replace the stock Merc cast iron exhaust manifolds with something like a Stainless Marine aftermarket aluminum exhaust manifold system which will help the engines breath even better. Either way, it takes the right combination of parts to get the right results. Do this and you may see 475-500+hp from a 454.

Call RMBuilder/Bob Madara....he can help you with your cam app and engine combination.

Have the heads checked so they are not cracked between the seats. GM rectangle port heads are notorious for cracking between the seats at random.