Are you saying the stringer is not glassed to the hull? hmmmm...
the stringer needs to be. If a stringer shows signs or rot, Id fix it. There have been plenty of threads on here about that and how to go about it yourself. With some patience and a little practice you could put new stringers in and KNOW that the boat was solid.
Stress cracks are there from abuse most likely, although they could be there because of inadequate bracing for the deck. There have been a few threads on that as well. To repair them, you need to grind them out completely, and lay some resin/fiberglass mat in place -obviously that kills the paint job. If they are not bad, you could forget about em.
the 455 is not the best motor to have. they suffer from inadeqaute oiling and unless you know what has been done, id probably write off going that fast with one-its likely to end up in disappointment and youll probably need a rebuild. The big block Chevy and Ford motors are a better foundation for going fast as they will rev better and are cheaper to build and have fewer design flaws for waht you want to do.
The WJ is a decent pump. Its not a berkeley and is therefor misunderstood by lots of people. Dont let the haters get you down on it. Mine will be doing 100 plus soon...
To go fast costs money. Have you considered swapping motors? Dont get me wrong, your goals can be reached with the olds/jacuzzi, but it will cost you. Even then, for similar money you could be going faster more reliably with a Chevy or Ford. The mods that need to be done cost about the same as they do on the other pumps.
Millers are good boats. All boats will deteriorate if abused long enough. If you hang out in the V drive forum long enough, Jim miller and his son Garrett post once in a while. Check it out.