I want to put ghost rings on my vent tube...barrels been cut down to 18.5......will I have to cut back the rib for the front sight or can I still mount it to the rib or what options do I have???

Probably depends on what style/brand of sights you want to use...

A set of GR sights like the Wilson Combat / Scattergun's have a front / rear sight matched in height and designed for the front sight to be mounted on the barrel. If you put that front sight on the rib it will be too tall.  I have seen that style of sights  installed on VR barrel by cutting back the rib so it butted and the back of the front sight (with the sight mounted on the barrel).

Various custom front sights mounted on the rib are an option with a bit of math and ingenuity.

True glow offers a combo that mounts on your vent rib, you can choose between rifle type or a ghost ring rear sight. I had a set mounted on my 3 gun m2 for a while, liked the setup a lot but its not as fast for blazing through targets as simply using a fiber optic front.

combo

I had Vang Comp cut down a 870 Super Magnum barrel he was doing his treatment on for me, and he incorporated the front ghost ring sight into the vent rib.  Looks very nice!  This was years ago and I don't have pictures, but here are some examples of others.