Regarding your question about designing a frame rail for flame cutting.....most of the laser cutters will be able to take a cardboard pattern that you made and digitize it so they can cut the shape. This should make it easy. Your data says that you are in Hopkinsville....if that is KY, I am not familiar with local laser cutters but you should be able to ask around. A sheetmetal shop usually has a laser but those lasers are always powerful enough to cut thick stuff. However, that shop will know who to send you to that can cut thick stuff.

Regarding the narrowing of an axle, I would say NO....do not do it. Axles are usually forged and require very specific welding. You will get 1000 replies that say "I welded one and it held fine" but the dead guys who had an axle break didnt see your question. Consider another solution....find a narrow axle, have a tube axle fabricated, etc.

Another thought is to plan your build with respect to tires. You should be able to estimate the vehicle weight (heavy) and figure out what is available in the tire line with a good load rating with margin. For instance, if you car is estimated to weigh
6000 lbs, each tire would see at least 1500 lbs (the fronts probably more). Using a 50% factor, you would need a tire with a load rating of at least 2250 lbe....maybe more. That rules out those 6.70 x 13 tires you had your eye on.