Spike, with the truck mods I was assuming that'd be a given. The Exo cage though would allow them something to grab onto, so a full cage inside would be much better IMO. As for the 6.5, I was under the assumption that it couldn't run on anything less than offroad high sulfur? Something to do with the injection system.
the later year 6.5's had that issue. but even so, all you need is a 7 micron filter and vegetable oil and you are good to go. just remember, in winter time, older diesels and bio fuels have gel issues.
the exo cage could allow them to grab hold, but ideally, that should only happen if you are standing still. I dont think a zombie will have the capability to grab (let alone hold onto) a moving vehicle, cage or not. now, post-nuclear era humans? probably.
and a full cage inside can restrict space availability. and the exo cage will do more to protect the vehicle in the event you have to ram something or grind the side of the vehicle.
Just imagine trying to escape a group of zombies and you swing down a narrow pass and bang the side of the vehicle. now your drivers side doors wont open... now you've got an extra hassle/delay egressing the vehicle
My primary concern is ammunition availabilty.
As far as Rebar bolts go, I'd fire them from the crossbow only because of the draw strength required. However as I further think about it; I could get away with broadheads for small groups (rebar being too massive in some cases). That or a barge/gaffing pole (allows me to keep distance and attack from two directions.) Problem comes in hordes or raider parties.
go fire some rebar... 
Answer: Brush cutter/Hedge cutter attachment on the front bumper.
Up armor the radiator housing with say, some
tin siding, attach brush guard with low slung serrated spikes.
... you can find sheet aluminum that is thicker than tin siding that will provide a little more strength than that. or even sheet steel.
With enough torque, smash my way through brush and create paths if roads are blocked, also use them to plow through masses of people/zombie hordes. Find a rancid water source, ford it, clean truck. Whatever's left hanging on get the barge/gaffing pole.
Pole could also be used to pull down fencing and collapsed barracades to get at potential supplies as well.