That XE268H should make tonnes of vacuum. 15.5 inches at 850 idle. And it's a known torque maker. Lots of guys drop those in 360 truck engines to make good bottom end torque.
I'm surprised at the power you're making. Your DCR is 7.85. That cam's not bad. Decent heads. Yet you're only making 280? Usually that cam is good for 1hp per cube. You should be making way more torque as well. That cam is a torque over horsepower maker.
I'd say you need a good tuneup. Check ignition, timing, carb, etc etc. You should be making more power. And shouldn't have a problem running vacuum anything.
As reference, I run the XE274H in my 340. And have no problem with power brakes and steering.
