Plugging fuel filler hole

[eric8]

I will be doing an LS swap soon on my 72 C10 so obviously the factory tank is going to be taken out. I'm trying to avoid paint or bodywork to fill in the hole left behind where the filler neck passed through. What I had envisioned is using a rubber type "plug" or blank off that attaches like a firewall grommet. I was curious if anyone knows the diameter of the hole that is left in the cab once one removes the filler neck and factory grommet?

I had also considered welding in a "dummy" filler neck so I could put on the stock gas cap and give the appearance of a stock truck from the outside. Has anyone ever done this? Stupid idea?