It would help if you stated what rearend you have in your car. It could be a 7.25 or 8.25 or 8.75 or Dana 60, etc... If you don't know which rearend you have, take a picture of it and post it up.
True to a point. The 73+ cars could also still be small bolt pattern in order with front drums and would only come with a 7.25 rear. That said there is factory large bolt pattern 7.25 rears.
If you are using the motor for drag only and are constantly ripping apart the motor and rebuilding it, go with the bronze. Otherwise, get a cam that has an intergrated cast iron gear on it and use a hardened intermediate shaft (it what my 340 has in it).
It's called a suregrip unit. Also if its just the unit itself, it will bolt into the either of the 741,742,489. If I were you I'd look for a clutch type unit.