You can hook up your heater with the small block outlets, but it is tight back there, and there is a good chance of the engine hitting the hoses and breaking the heater core. The big block cars use a different heater core with longer inlet/outlet tubes and the core is flipped 180 degrees so the hoses go into the actual core cover, instead of next to the cover on the small block. If you are bucks up, it looks nice to have the factory style stuff but its not needed. First you will need to get longer tubes put on your heater core, a radiator shop can do this. Then install it flipped 180 deg in the car with the cover off, mark where the tubes will come though the cover and cut the holes. Put a small plate over where the old firewall holes are, and install the cover. This works fine. Make sure your core is good before changing the tubes, if you need a new one just buy the BB core to start with.
66/70 chevelle ss396's