Air Filter Question

I'm new here, so please bear with me!

I have a '99 F250, Extended Cab, PSD, 2WD, that is my daily driver. I drive it around town (empty) in my business and tow a 36' 5th wheel toy box when traveling. Bought it new, and currently have 147,000 on the clock. Have had a Hypertech Programmer in it for nearly the whole time.

About a month ago, I cut out the muffler and straight piped it. Added a bit more zest and increased my mileage.

I am looking for a bit more zest and a bit more mileage again. My son has suggested the K&N Air Filter system. According to their web site, it would add 8+ horses. I do not drive it on dirt, unless the streets are being worked on, so I'm not concerned with the dirt pick up issues from off-roading. What should it do to my mpg?

Next question, I was looking for a slightly larger fuel capacity. Again, my son who is in the wrecking yard business, thinks that I could take the 36 gal tank that comes on the dually and put it in my truck. Will it replace my 29 gal tank and fit without some crazy modification?

Thanks in advance.