I run 215/70/R15 Radial TA's on my 87 Comanche with the same driveline as your truck. They're on the factory alloy wheels. Been on the truck for over 30 years. Not the same tires the whole time.
I traced the hoses today and the one that goes to the VAC port does indeed go to the intake manifold in a round about way and the other ports hose goes through the firewall into the cab.