The HV stuff is designed for the bigger 1/8 scale cars. They have huge torque numbers but they get the speed from the fact that most are using a minimum of a 4S lipo. HV motors and the Ballistic that you meantioned don't really compare, they weren't meant for the same application. Second a 5.5 BL motor in a Slash is simply INSANE!! you don't need that much power.
What are you looking to use for batteries? 2S, 3S? Are you going to race?
Personally if all this is going into a Slash I would get the HV stuff out of your mind because that's not really what it was meant for. I don't think the shaft sizes are even the same. I can't say 100% that it won't work though.