venting gas tank on ex-diesels

[abig84]

1986 k30 diesel now has a 400 in it tank does not vent.

I remember after I built this truck drove it to the gas station and the gas tank had a large vacuum. Hissed like crazy after cracking the cap. So my way of fixing it was drill a small hole in the gas cap. Problem with that is can't ever fill it over 3/4 tank or it splashes out. So 10 years later I think it's time to fix it right

So anyone run across this before? Is there a vent on the diesel trucks that is maybe clogged or do I need to change sending units? Probably need to change filler necks also as this 3 inch hole that comes with the cucv trucks sucks for filling up