The way I do the base frame is on a large 1" thick steel table and clamp everything down on quardinate and weld 100% prior to releasing. The table has windows in it for this purpose but you do not need to have a table. You must have a strong back of some sourt or a jig makes the job easeyer but not necessary. Just make everything perfectly level and square. No biggie.
As for holding the body in place over the frame. I weld splines or pipe pcs to hold it on dimension to the frame so I don't bump it during working while it is on the shims over the frame, and the shims you use to level can be any variation you choose and feel will hold everything in place most fab guys use steel shims but wood is just fine too.
As for the pipe quantity's that will depend on how you make the double frame rail and what size pipe you choose to use weather it is 1 1/2" or 1 5/8" and so on. Here is a preliminary pipe schedual for getting started.
A stick is a random length 20' piece of pipe
6 sticks + of 1.625
4 sticks + of 1.5
3 sticks of 1.375
2 sticks of 1.25
2 sticks of 1
2 sticks of .75
2 sticks of .5
The smaller tubing like the 1.25" and smaller I would use minimum wall thickness NHRA allows, and the larger size pipe you will need to determine what your bender can use effectively with out necking the pipe or putting a ridge in it.
I would get a few square feet or 1/8" 4130N plate for brackets and one sf. of 3/16" 4130N plate. The better you are at making brackets the less plate you will need. If you are making your 4 link brackets too you will need 1/4" 4130N plate but make a templet first so you know how large a piece you need to cut them out of.
This will get you started and possibly finished. The more pipe you have in stock the better you can do bending the pipe. You will be braver knowing that you have more in stock for errors. Error bends can be helpful sitting in the corner for other parts like the funny car cage etc.
Good luck with your project
Bruce