Putting larger tank on a 81 Specal

The tank on my 81 Special needs a dent fixed and repainted. Instead of fixing this tank, I am thinking of acquiring an older tank which will increase fuel capacity.
A few questions are, how does the double petcock work? Is one ON and the other RES?
Can I swap gas caps, so I can use the same key?
Are there any fit issues I will need to deal with?

Ok, the manual pet cocks have three positions. Off On Res. this is the same with to petcocks.
If you look at your tank there is a central tunnel This tunnel separates the left and right sides of the tank. At the front any way.
Near the rear it gets pretty much the same.
I'm not sure why they used dual petcocks unless they thought that one wouldn't flow enough.
I'm not sure about most folks but I only use the left side petcock. The earlier carbs got fuel from the petcocks but they also had a fuel crossover to link fuel flow. This is more than sufficient to feed the carbs using just one petcock.
Using an earlier tank to gain the extra gallon of fuel is a good idea. You only need to use one petcock. You can leave the unused petcock and plug the outlet or remove it and install a block off plate where the petcock was. They sell then or yu can make your own.
Gas caps won't swap Your cap is mounted with a single screw. The older tanks, the cap is pined through a block on the tank and the lock catches on another block on the tank
It's not hard ti remove the pins from the locks. this lets you open the cap with most any thing but still looks like it needs a key.
My 75 tank is that way. Never had any issues with it. My Harley doesn't lock at all. Never had any issues with that.
Not enough fuel to be worth the effort to steal. Never really understood the reason for thr lock.
On all the 80 up tanks I have the key locks didn't work well. I replaced the lock slide with a longer piece of strap steel bent up just in front of the cap. Just Enough ti give me room to push it to open the tank.
There are threads about this on here somewhere. Just search for it.
Leo