I'm with KW on ths one, you're not building a high dollar hard at it compitition race car. I would build a rec. tube base frame, and you can buy the whole kit for about $600.00 then use .134 wall MS, for the cage and all the bracing. This will put it all under your control in the shop and you can weld it. If you screw up on a bend or notching, you have control of it and can fix it.If you go to cromoly and take pieces to your friend and something does'nt fit then it becomes a run around. Besides if you're doing it for your students, they can learn every facet of the build when it's done in house. A rec. tube chassis will certify to 8.50 easy and to 7.50 with a few more bars, and your students can say we built it, not like if you sent it out to be welded and fitted. By the way i took auto shop 45 yrs. ago and got hooked on cars and racing since then.
They had a highschool build off with 2 highschools a couple yrs. ago, one school did exactly what you're talking about, they sent the chassis out and had a chassis shop build it the other school did everything them selves and the school that did everything themselves finished their car first, and not only did it run faster but when the high dollar tube car got finished it still got outrun, this was in eastern Tn.
JMO
Zip.