K - first off it looks like that "3 speed" only refers to having a shifter lever for an existing 3 speed rear hub (like on a cruiser bike) and has nothing to do with engine gearing... I don't like how that site worded it to sound like the engine itself has gearing, it isn't a centrifugal it's a "friction plate/dry" same as the rest... so there'd be no advantage to havin' that one on yer 21 speed bike
I'd skip them and go w/Dax, such "clever wording" in advertisements kinda pisses me off tbh.
Secondly... there's no real way to mount an in frame kit on that bike in particular as there's simply no
room at all, my build has a similar frame style for the top tube at least and even though it has a LOT more room than yours it was a very tight squeeze.
I'd say you have a coupla options. You could look into rack mount engines or electrics (both quite costly, starts at about $400), you could get a bike better suited for an in frame kit - and/or you could find a donor frame and swap out the better components from the Giant, which is prolly the cheapest route.
I know they're crappy, but many Wallmart bikes actually have really nice frames (look at the welds to judge quality) but they cut corners to lower the prices by bolting on the shoddiest mechanicals possible. You could get one and swap out all the really nice bits from your Giant. I'd go with that method if you can't find another bike/concerned about yer pennies.
Sorry to be the bearer of bad news, but all is not lost - just a minor setback is all. Keep us posted and good luck in yer quest
Edit: lol, I just re-read yer OP sorry man
- sounds like you've the right idea after all! Ya got any pics of the other frame?
This is the bike I started with and although I had quite a bit more room, it still wasn't enough and was a bit of a nightmare lol (the "dropped" top tube is sneaky) so I'd advise a standard (preferably steel) V frame for a first build;