Both above & below show tube preperation prior to welding insert into place,

by angle grinding a finishing with a file... for this example i cheated by milling it out.

Mustang Gas Tank
3.3 U.S. Gallon Single Cap
Ideal for any custom Bike,Trike, Chopper or Bobber
Raw Steel Low Tunnel universal mounts converted to our rubber mounting system  
The tunnel measures approximately 2 1/2" High x 2 3/4" wide & approximately 19 3/4" mounting tab to tab end. With 18 1/8" ( 46cm ) fastener centres, the tank itself measures around 17" long x 12" wide.
Requires 1 Cam Lock style plain vented gas cap & excepts either two 1/4" BSP petrol taps at the rear or a 3/8 NPT midway on your right hand side when seated.
Now here comes the disclaimer part....as the manufactures states... pressure test and use a sealer/liner before use.
As always it`s best practice to do your own preventative measures, for a trouble free life... May have slight dents & imperfections and will require preparation for final paint.