Need a little help. 2 questions

[Baggerjohn]

I have a 67 lwb 2wd C-10 with a 250 inline 6 with a 5 speed trans. I don't know what rear end gears I have. I am getting really bad gas mileage. I do not have a speedometer. I have the one barrel carb. I had hoped for better mileage. I do have a tach that shows when I'm going approx. 65 mph the tach is at 1200 rpm. Think the tach is wrong. Any help would be great.
Question #2 is the bench seat my truck came with is touching the doors when closed and it is uncomfortable. I bought another seat that I think is the right length but when installed sits real low. How can I safely raise the seat so that I see where I'm going?
Since retiring my wife thought I needed something to occupy my time I chose this truck. Just need a little advice please. Thanks.