I ride plate alone isn't the answer to a porpoising problem on the 440/550 hull. I went through this with my old 550. Yes, an extended rideplate will help, but it will never cure this problem. On these hulls it's all about the rider's technique. You have to learn to work the the ski and learn to absorb some of the hulls up and down movement to keep it hooked up to the water. If you ride with stiff legs, you'll never get the hull to stop porpoising.
Any rideplate longer than the stock plate is best. The stock plate on my '86 550 was flush with the back of the bottom half of the hull. My favorite ride plate was from Ocean Pro. Ocean Pro made 2 different ride plates for these hulls. The "Ocean" is a more aggresive plate with extra fins that help the tail end from sliding out in the corners, but again, the tail sliding out has more to do with not leaning the hull into the corners enough than the right ride plate. Here are a couple eBay adds for the ones I'm talking about.
My favorite:
http://cgi.ebay.com/ebaymotors/OCEA...9597755QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear
The other option:
http://cgi.ebay.com/ebaymotors/OCEA...90020826205QQptZPersonalQ5fWatercraftQ5fParts
Depending on your weight, there are a couple other things you can do to help you out with the porpoising problem.
1) Try spacing the rear-most ride plate bolt holes off the bottom of the hull with a couple washers. Put the washers between the ride plate and the hull then bolt it up. This will help (although minimal) trim the nose of the hull down into the water more.
2) Depending on your weight, you might want to look into finding a pump spacer. This aluminum spacer wedge goes between the pump body and the exit nozzle on the pump to angle the nozzle downward which again helps to drive the nose of the boat down. This item won't fit the 550 pump, but it shows you what the part looks like:
http://cgi.ebay.com/ebaymotors/Kawa...90089428887QQptZPersonalQ5fWatercraftQ5fParts
*** Like I initially said though, porpoising on these hulls is mainly resolved by rider input and skill level. These hulls are proned to porpoising a lot more of the more modern hull designs.
EDIT: I don't know if that you riding in picture beside your screen name, but it's really hard to ride an old 440/550 hull while standing with both legs together at the back of the ride tray. I found you mainly need to have a surf or slalom waterski type stance to keep your weight forward. Try to keep most of your weight on the front most foot and absorb those "bumps".
I was hoping a full gas tank would help with it, but it didn't seem to matter the last time I was out.