I just bought one used and I'm not sure exactly how it installs. I didnt test fit it to the car but it seems like im going to have to drill my own holes and make some sort of backing plates beneath the car. If anyone has installed one or has experience w/ other bolt in cages I would appreciate some assistance.

Also would their be any benefit to weldng in a bolt in cage after its been bolted into position?

 

Yes, you will have to drill holes and make backing plates.
Be very careful when drilling that there is nothing underneath (like brake/fuel lines) that could be damaged.

As for welding...I'm not sure if it's worth the trouble.
Seems like bolt in bars would have to be designed so the mounts are placed where there is access from the underside- this would typically be on a sheetmetal area. Thats why the scab plates are necessary.

A purpose built, welded in bar would be able to tie to more structurally sound areas since underside bolt access is not an issue.

Welding your cage probably wouldn't hurt but I doubt it would add enough extra rigidity to really matter.
I'm no roll cage expert though, so a modicum of salt is in order here.