Hello Jan.
The 75 inch span Sealand was designed for larger Magnetic Mayhem motors. You will notice that it is longer and has a much larger wing area, 830 sq inches as compared with 695 for the Sealand 480. The wing chord is quite a bit more on the 75 inch wing but it is not really true to scale. To be true scale, the wing of the larger model should have been over 80 inches, but for ease of transportation I shortened it to 75 inches and increased the chord.
Last year the Magnetic Mayhem motors were discontinued here, so I scaled down the Sealand plan for Speed 480 motors. This shortenend the fuselage and brought the wing span down to a span of 74 1/2 inches if I followed the proportions of the full scale Sealand. This is a convenient size for transporting, so the smaller Sealand 480 is more accurate in terms of scale when comparing it to the full scale flying boat. You can see more details on the sub page "Model specs" on my web site.