You are on the right track. Do not start cutting holes in your jeeps floorboards, there's no need and that method will just take twice as long.
If you can't snap the heads off the bolts, use your angle grinder or a dremel with cut off disc to cut the heads off flush with the bar-pin. If it's too tight to get those tools in there, start drilling the head off with a big enough drill bit. You might not even need to drill the entire head out, just enough to weaken the metal then get a socket around it and torque it until it snaps.
It doesn't have to look pretty because you'll be replacing the hardware anyways. Just do what it takes. Once you have cut off the heads, remove the shocks completely. At this point you'll be looking at the remainder of the bolts which are rusted to the inside of the jeeps "frame". Lets call them the studs from now on.
I used a large center punch and 5lb hammer to knock the studs out. I had a hard time getting the center punch to stay centered while I hammered on it, so I drilled a shallow pilot hole with 1/8" bit in the center of broken stud. 2-3 solid whacks with the hammer and they popped off and I was left with clean mounting holes under the jeep.
Next, you can either feed new grade 8 hardware (I believe I used 3/8 x 1" but can't remember) by using the wire method through the access panel OR by taping your new bolt to a wrench and fishing it through the access panel that way. Either method will work and it will only take 5 minutes, gauranteed.
I had the heads snapped off, hammered out, new hardware fished through and both rear shocks installed in less than 45 minutes. Only advice I'd give is wear eye protection since there's alot of dirt that gets knocked off when you're hammering on the old shock bolts.