what kind of HP gains can i expect from my 97 supercab with a methanol injection system?

 

You mean naturally aspirated? You'll lose power....

 

Unless you've got other mods to suport it that is.

 

Methanol has no magic qualities as far as fuel heat goes.
It has less than half the heat content of gas and requires about 1.6 or more times as much volume to run at the right A/F ratio (about 6.5 to 1).
The small gain might come from it's ability to cool the charge coming in.
It is way more effective on carbed engines where the intake tract has a longer time to cool and reduce density due to where the fuel is metered in before the throttle plate.
The larger volume needs make mpg go way down and needs a fuel system to pass that extra volume and be corosion resistant and not have deteration effects.
Cold temp starting can be a big problem in winter.
I run a sprint car racer on it and have to drain and clean the fuel system after every trip to the track even with all parts built to pump the fuel.

 

You can build an engine to run on methanol or ethanol and get decent results if you really build it right, but the question as stated was methanol injection, which would be on top of gasoline, and which without a blower there is really no benefit. Without tuning you would lose a bunch of power, and even with electronic tuning you would be hard pressed to even get back near your starting point (before injection)....