In cab fuel tank
 

I'd like to put a tank in the cab of my 1977 F150. Found a couple for sale on craigslist for sale. My truck has only a rear tank under a flatbed and is really hard to put gas in it. My questions are, what size hole do I put in the cab to run the fill spout? Does it bolt to the rear cab wall?

That was a REALLY-bad idea even when it came from the factory. Adding one isn't safe, and probably not legal, either. If you need more fuel capacity, put a tank in or under the bed, or add Jerry-can holders. But I'd think gas stations are easy enough to find in PA that the stock tank would do.

If the filler neck is the problem, fix that, or put a normal bed back on.