its been a while since u posted this but here goes if you still read it!
a wah and depending on what mxr pedal maybe phaser or something you want to use generally go BEFORE your amp...
however you can experiment and put them in the loop either before or after the g major.
i always thought that wah sounded best directly after guitar since you want the signal before it is touched by any eqing or pre amp coloring to be "Wahed" already
but it sounds totally different in the loop because a preamp conditioned signal is getting wahed.
try both see what you like most. just understand conventionally people put wah before amp. but then again conventionally people do unconventional things. it;s a matter of taste. by now 3/4 of year after your post hopefully you figured it out if not then better late than never!!!
cheers