I have built a lot of Jr dragsters, had a shop for several years. Get an NHRA rule book the cage must be built to specs. The rest of the car you can do pretty much what you want.
Go to ARCRACING.COM you can get all the parts you need from them.

I would build it from moly for resale value, it must be tig welded. The cost of the tubing is not much more and the frame will be about 20 lbs less. 20 lbs is 2 tenths in ET.

There are several Jr Dragster forums JRDRAGS.Com and MAINEFAMILYRACING.com good folks and can answer a lot of questions.