If the rear and torque tube is still intact, just measure the angle the factory gave it. That will save a lot of calculation. Angle finders can be bought for less then $25 at building supply places.

I would remove the rubber because it will it will burn. This will not only stink, but it could catch something on fire and you can't see what you are welding.

I have a some polished stainless steel that I use as a heat shield when I weld. But to be on the safe side, I would remove the gas tank. There is no sense in taking unnecessary chances.