If you build it like the factory did a 383/auto you won't need a cross member or a rear sump pan. If you have no pan then a rear sump is probably easier to find. The driveshaft from a 383/auto truck would be the correct length, and since the crossmember is in the same spot for all trucks, perhaps the slant 6/auto driveshaft is the same length as well.

I don't know that the radiator has the inlet/outlet in the correct spots, but if so, you may be able to get by with it. I do know some with big blocks said their trucks ran hot. They don't make radiators for these trucks but apparently a Charger radiator is a close fit. Again you will have to check where the inlet/outlet is. http://www.championradiators.com/Dodge-D100-D200-1961-1971-radiator

The suspension should be fine if it is in good condition.