Years ago, I built a sport 20 Pay'N Pak, starting with a Dumas kit. Since I didn't want to build it using the die "crushed" mahogany framing, I spent several weeks reworking parts and changing into a hull that would work. I even documented the process in a thread linked below:
Re-Engineering A Dumas Pay'N Pak - RCU Forums (rcuniverse.com)
At the request of a new builder from the land down under, I've agreed to do a thread on building a 1/8 scale 1973 Pay'N Pak. Like the sport 20 from last time, I will be making several changes from the plans based on measurements taken from the full sized boat, presently on display at the Hydroplane & Raceboat Museum in Kent Washington.
Before I actually get started on the build, I'm going to give a bit of information on the plans. The plan set was originally drawn by Roger Newton as the 1974 Miss Cotts Beverage. The full sized boat was originally the 1972 Notre Dame, so changing the plans to make any one of 39 similar boats was basically changing the color sheet that showed the paint scheme and the configuration of the cockpit, cowling and tail arrangement. The "Pak" was the fourth plan set drawn and was, however, different than the rest. The designer, Ron Jones, made FIVE major changes when he started working on the "Pak" from the previously built Notre Dame:

• The transom at the back of the boat was flat topped while every other boat was arched upward a few inches in the center
• The hull and all of the framing was built primarily from 1" thick aluminum honeycomb instead of wood. Only the deck and curved sponson skins were made from plywood with fiberglass on top
• The cockpit, cowling and tails were welded aluminum instead of the normal molded fiberglass
• Instead of a single centerline vertical tail faired into the cockpit, he installed twin vertical tails with a horizontal wing mounted on top
• The cockpit was going to be in front of the engine in the now common "cabover" configuration. This last part was vetoed by the person buying the boat, Dave Heerensperger, due to the fact that it didn't work on the previous twin "Hemi" powered 1970 Pak, but that's another story

Due to these changes, Roger modified the plans to make the parts usable to build the Pak. He did, however, make some compromises that I will be fixing as I go.
Now, it's time to start the build.
The first change is the bottom of the sponson insides. Using measurements taken off of the full sized boat, the depth of the sponsons has been reduced from the plan depth and the air traps reshaped to match the full sized boat.
Change number 2 is the height of the transom and the deck shape of the rear of the boat. The plans have the transom shorter than the scale height. The transom on the full sized boat is 13" high, thus making the scale transom 1.625"(41.275mm), .25"(6.35mm) taller than shown on the plans. This requires redrawing the top of the engine bay walls and the sponson insides to make the corrected transom fit properly.
Change number three is I have to move a full width cross frame forward roughly an inch(25.4mm) to get the firewall at the rear of the engine bay into it's correct scale location and add a second bulkhead at the front of the engine bay where there is one on the full sized boat that supports a large oil tank used for the lubrication system in the Rolls Royce Merlin that powered the boat.
One change that I didn't have to make was the bottom of the engine bay walls. I went with a 4 degree rise at the rear of the airtrap section of the sponson insides which, surprisingly, matched the plans. It was also very close to the full sized boat which has a 5 degree rise.
Now that I've bored you all to death, it's time to cut out the lengthwise frames and add stringers to the bottom of them so there will be more to glue the bottom to. Pictures to come within the next few days