You will need to drop the fuel tank, you "might" be able to get the nuts on the new bolts and then hold them, but the time it will take for 4 bolts would probably be more than the time it takes to drop the tank.
Removing the bed makes access and working on it much easier, but tank still needs to be moved.
If you do remove the bed then I would replace all the hangers at that time.
You need to grind off the rivets, so watch the sparks near the fuel tank.
The bolt on the old hanger will most likely not come out of the bushing, I ended up having to cut off the bolt on each hanger and then used a propane torch to melt the rubber on the bushing enough to pound it out, the Black Smoke is AWFUL!!!!!
This generates a lot of heat and will blister the paint of the bed above so wet it down constantly, or remove the bed.
If the bed is off it would give you a better chance to get the old bolt out but..........old bushing may wear out and then you have to do it all over again.