The I6 is longer than the V8, but both are mounted centrally to the engine compartment. The 300 is a better truck engine because it develops it's peak torque at a lower rpm than a V8. Part of the reason for this is the huge pistons the 300 has, but that's a long story I don't really feel like going into. What you need to know, and can correct your friends by telling them, is that the 300 reaches it's peak torque, the point when the most raw power is available, at around 2000 rpm. The 302 doesn't reach it's peak torque until around 2400 rpm. That means that the 300 will get it's power sooner, and with less effort than the 302. That makes the 300 a better truck engine. If you want to go fast, and don't really care about power, then the 302 would be a better choice because that engine can rev higher, and higher rpms equal speed.

There is a lot more to it than that, but that's what it breaks down to. Now you can go show off to your buddies.