I think the 350 (Ampeg SVT 350H, right?) runs at whatever the impedance that the load it's driving is, it's not like a real SVT that matches cab with tube output impedance using a tapped transformer, the 350H is a transistor power amp. You have no choice over "what impedance i should run the heads at" other than by what impedance the cab(s) are that you plug in.
You don't want to connect a total load under 4 ohms, or it may burn itself up.
What is the impedance of the 215 cab you are planning to use? Should be on the cabinet somewhere, or you can just measure the cab with a multimeter and find out. Most factory 215 cabs are 4 or 8 ohms, but testing it would be a good idea.
