Me ears are burnin' Blakenstein
Phantom - pics would help, I dunno what a Schwinn Phantom looks like but I will say that if there's any question of the motor fitting properly you may want to consider a different frame for a first build. While with time & patience ya can stuff a motor in just about anything - the consequences may be too much to deal with for a first build.
It took me over a month to get my bike together because of clearance problems, I love it now that it's done - but my friend's kid built his bike in three days and he's only 11 years old! The only difference was with his bike it was a direct bolt-in and mine required... ingenuity.
If you pick up a frame that doesn't require engine/carb mount mods - you can always use it later in that frame you like so much once you've figured the peculiarities of these lil motors...
I figure the point for most of us is just to ride the things, "extreme" mods and builds can come later - but feel free to PM me ifn ya wanna.
Just the tip of the proverbial iceberg with the headaches involved with a frame that's too small/wrong shape - and it's sketchy in some ways;