I did some on-line searches and won't point to anything specific that says how to secure your welding gases to your truck. You can read them and draw your own conclusions if they are applicable.
a couple of excerpts:
1926.350(a)(4)
When cylinders are transported by powered vehicles, they shall be secured in a vertical position.
1910.253(b)(5)(iii)(A)
Fuel-gas cylinders shall be placed with valve end up whenever they are in use. Liquefied gases shall be stored and shipped with the valve end up.
A couple of informative sites that may not apply, but interesting never the less.
http://www.workcover.nsw.gov.au/NR/rdonlyres/92F40BF9-4126-4C82-9FDB-33DC6CE21E1D/0/transporting_small_gas_cyclinders_966.pdfwww.airproducts.com/nr/rdonlyres/9d325c49-7c62-41e5-aa0b-8411db4d84f8/0/safety13.pdf
http://hazmat.dot.gov/ (some proposed rule changes)
PART 177--CARRIAGE BY PUBLIC
HIGHWAY
41. The authority citation for Part 177
would continue to read as follows:
Authority: 49 U.S.C. 5101-5127, 49 CFR
1.53.
42. In § 177.840, paragraph (a)(1)
would be revised to read as follows:
§ 177.840 Class 2 (gases) materials.
* * * *
(1) Cylinders. Cylinders containing
Class 2 (gases) materials shall be
securely lashed in an upright position,
loaded in racks, or packed in boxes or
crates and securely attached to the
motor vehicle to prevent the cylinders
from being shifted, overturned or
ejected from the vehicle. A cylinder
containing a Class 2 material may be
loaded in a horizontal position provided
that the cylinder is designed so that the
inlet to the pressure relief device is
located in the vapor space and the
cylinder is properly secured and lashed.
Best regards - Al