I got a 250 miles on the engine then noticed oil on the tailgait. My air box was full of oil. Why would oil be sucked through the tube at the base of the filler spout?

 

Too much Blowby

Blowby occurs on all piston engines
When a cylinder fires that explosion pushes the piston down to power the crank/vehicle
And when that happens some of that explosion "blows by" the piston and rings and into the crank case, hence the name "blowby"
The HOT gases from the explosion will Vaporize some of the oil on the cylinder walls and sides of piston, this is the ONLY place in an engine hot enough to vaporize oil, just FYI

Most of the oil vapor will condense in the crank case, but it is HOT "air" and "hot air rises" so some of the oil vapor will go UP the oil drain holes in the heads and into the valve cover areas

PCV(positive crankcase ventilation) system was added back in the 1960's to re-burn the Blowby gases and any oil vapor
So there is a PCV Valve hooked up to intake manifold to suck out Blowby and oil vapor and re-burn in the engine
On a V6 or V8 engine PCV Valve will be on one valve cover and a Breather/Vent hose will be on the other valve cover(can be on oil filler tube)

If you have oil vapor coming out the Breather then either the PCV Valve is unhooked or broken, or the engine has way TOO MUCH Blowby
Oil vapor will condense back to a liquid pretty fast in the Air Plenum(big air tube from air filter box to upper intake)
And if air plenum is at an angle, the now liquid oil, could run down into the air filter box

Rebuilt engine should have very very little blowby

With engine idling pull out the dip stick, there should be very little air flow coming from that tube, in fact air should be flowing INTO the dip stick tube if the PCV system is working like it should

As an engine gets up into the 200k mile range blowby will go up just from normal wear and tear on the rings, but not even close to what you are seeing