Ok oldest pics & info are below. When I first started hacking on this tank. [About three years ago.]
I used a drill to "cut" it open. Now with much better tools I have revisited this tank mod out of boredom.

First thing I did was cut it into 3 pieces.
I cut both sides off and then cut out the old mangled tunnel.
 

Made a new tunnel and welded it off.
Tunnel is double welded but not 100% done.
Gonna grind everything smooth on the inside and use a special metal filler to smooth it out.
 / seal any pin holes.
 

Tank has rusted since last work on so it doesn't look good.
But its only surface rust and I have chemicals to fix that.

First I beat the heck out of the inside with a 2x4 to get it formed as much as I could that way; then I split the bottom to go even farther.
I also notched the  top neck of the tank for a better fit.
 

You can see how the stock tank hides allot of stuff.