Re: Question on proper wheel/tire size.
I tow a 15 foot Glastron with a 90 on the back with the same size tires. BANG. BANG. I get a bulk discount from Wal Mart. The reason I have not changed size is because the trailer and fender mounts will not allow it, and now I rarely use that boat. Those little 4.80 X 8s have a very low capacity--something like 750-850 pounds so two are going to carry not only the boat, but all the crap you put in it AND the weight of the trailer. You are probably near or at capacity now and with the larger engine will probably be overloaded.
If you have the option, upgrade to at least 4.80 X 12 or 5.30 X 12. Tires will generate a lot less heat and last much longer, and definitely be safer.