If it flew with a .40, and it's an old plan, it seems like you're just asking for complications. A K&B .40 made about 0.8 HP. Currently .40 size planes are frequently fitted with a .46, and they make almost twice as much power as the old K&B. I'd look at the new OS 4-stroke which is a .51 if memory serves. Put the .90 FS on there and you have an extra half-pound on the front, so you need an extra quarter-pound in the tail to balance it, and you sacrifice a lot of the performance advantage for a plane that now has to be landed fast because of the extra wing-loading. Fast landings can be a disadvantage for a floatplane since you have to clear the trees on the opposite bank then drop in and land before you run out of pond.

Beavers look sooooo cool making scale takeoffs, running about 300 feet then gently lifting off. I had a Senior Telemaster with 8-foot wings and a .91 OS FS. I bashed it a bit to look like a Beev. (See page 122 of the December RCReport)
It only weighed 10 pounds and it would make a scale-like take off from water at less than half-throttle. OR it would take off in 8 feet if I goosed it.

With a 70" wing you have a much smaller plane. I may just be old-school but I don't really expect Scale utility planes like CUbs and Beavers to do 3D maneuvers. You don't NEED a thrust-to weight ratio greater than 1:1.