Two Bears,
Don't know what happened to the picture but it doesn't show up. Love to see it though.
The answer to "would I have enough wall thickness if I rebored it to golf ball size" could actually go either way.
The bore size needed to shoot a golf ball is 1.723" and your gun tube is 2.5" in diameter. If you rebored it, you'd be left with a .3885" wall thickness which is just over 3/8". That's not really enough metal for safety if the gun is bored full depth.
However, if it you went the Mountain Howitzer route and only drilled it out part way so that a powder chamber was left (.75 by maybe 1.5") then it could probably be OK.
Lets see what the other guys have to say.