The big question is how you would do seatbelts. In the 80, the D pillar is designed to mount a seatbelt. With an internal cage, you should be looking at five point harnesses, and I'm not sure that 80 seats could be modified to accept harnesses. But in my opinion, that is the only way to keep your kids truly safe in a vehicle that is used a manner that actually warrants an internal cage.
In any case, the current line of safety thinking is that both seats and belts should be bolted into the same structure. In other words, you either bolt both seats and belts into the vehicle directly, or you bolt both into the cage. The reason is that if you have seats bolted into a cage, but belts into the vehicle, and in a severe impact the cage is separated from the vehicle, then belts could either become useless or deadly. If everything is bolted to the cage, then your kids are protected in that their entire restraint system should remain attached to the cage under any circumstance (and the cage should be at least .120 DOM for an internal setup, although for adding seat mounts for my kids I think I'd sacrfice the weight and use a bit thicker tube).
You can get clip on harness mounts, and the question would be whether you can also do clip on mounts for the seats so that you can quickly remove the entire assembly for trips when the kids won't be coming along. This would be ideal.
I'll toss in a shamless plug and advertise that I have a set of preteen (good for up to age 12) racing seats from PRP that are designed with mounts to bolt (or possibly clip) into a cage, along with five point harnesses. I also have a set of toddler sized seats with harnesses (good up to about age four). I was going to put both sets into a Jeep Cherokee to fit all four kids while they are small, but am moving into a FZJ80 instead. All seats and harnesses are unused, and the harnesses meet SFI 16.1 certification. The seats are properly sized for smaller children, which keeps them properly belted and saves cargo room. I'd part with a set of seats and belts for $200, which less than half what they cost new.
This is a pic of the seats sitting in the basic position (they would be several inches higher and to the rear). I hadn't completed the cage in order to create the mounting points (c pillar hoop, seat mounting floor, harness bars across B/C/D pillars, reinforcing triangulation), so they aren't bolted in at all. PM me if you are interested in more details or the seats and harnesses.
Nay