What type of isopropyl alcohol do you have. The ones I'm familiar with are 50/50, 70/30, and 90/10. 100% isopropyl is also sold, but it's mainly for commercial/industrial applications and not readily available to your average consumer.
I would use the 5 gallons of methanol as the alcohol ingredient in mixing up my own WSW solvent. I would not advise pouring methanol into the gas tank as it has been shown to corrode certain metal parts found in modern vehicles fuel systems.
If the isopropyl is 100% pure, I see no harm in adding up to 16 ounces to a tank of gas as a combination gas dryer/anti freeze. If the isopropyl is one of the above mentioned consumer blends, I wouldn't use it in my gas tank. No sense ADDING water to your gas, even if it is a small amount.