8 ohms. What you're measuring is the DC resistance of the wire in the voice coils. The AC impedance will be higher. 7.6 ohms is not an unusual DC measurement for an 8 ohm speaker, though it is, in fact, a bit on the high side.
Most drivers are made such that a positive voltage applied to the + terminal (often red) will cause the drivers to move out. Having wired the cabinet yourself, you should check this. Apply a 9 volt battery to the wires, noting two things. One, that all of the drivers move in the same direction when voltage is applied, and which wire is connected to the + battery terminal when they move out rather than in. Connect that wire to the center post terminal of the jack, the other to the sleeve terminal.
The Sceptre was rated at 60 watts. Many folks say they got more like 80 watts. In general, I'd say you'd be fine with your 100 watt head. However, if you're playing with everything on 10 all the time, then there could be a problem with drivers that old, and 50 watts would be better. With drivers that old, though, there can't be any guarantees about anything.