I'm doing an LS conversion in a '71 car, would like to change to a big block heater core to get more clearance for the connections. Is it as simple as changing out the heater box and heater core or does the stuff in the inside have to be changed also?

heater core and heater box:yes::yes::yes:

There are aftermarket kits made to convert sb to a bb heater box. I convered both of my cars. If you do it this way the only big buck item is the bb heater core. Everything else inside is the same.

Thanks for the info guys, I was hoping it was as simple as that!