The first step is to identify if your welding needs to be done to a specific welding code or specification.
I assume that the main requirements you need to meet are DOT.
Going from memory, I don't think DOT calls out specific welding requirements, other than to state that welding needs to be done in accordance with an approved procedure, and Welders need to be code qualified. I think it calls out AWS and/or ASME, but my memory may be failing as I haven't finished my first cup of coffee yet this morning.
Others may provide better info in this area.
Second step is to compile and document the efficiencies of your current process, which will provide a base line to measure against.
This will help to provide comparisons to other welding processes, to help select the best options that provide both increased welded joint integrity and increased efficiency.
This will also provide an estimable ROI to justify the expense of different procedure / performance qualification testing and possible new equipment.
Third step is to R&D different processes to see what has the best potential for overall improvement, so you can make an educated decision.
Short Circuiting transfer use in structural applications is restricted in welding codes due to it's inability to provide reliable weld penetration / weld joint integrity, especially when welding vertical downhill. It's also a very slow and spatter producing inefficient process.
Spray transfer is great for weld penetration and joint integrity, but has little to no open root and vertical downhill capability, which means that joint preparations have to be tightly controlled and welds have to be positioned flat or horizontal. Spray also has limited reliabilty on material thicknesses less than .1875" thick.
Modern pulse equipment with multiple waveform capabilities is highly versatile for all material thicknesses and positions. When done correctly, it can provide much more reliable penetration and weld joint integrity than short circuit, even when used in the vertical downhill position, can be tuned to create little to no spatter, and can run efficiently at 15 - 30 IPM travel speed, which I'm sure is much higher than your current short arc.
My recommendation would be to look closely at pulse with either Miller Continuum and Lincoln Powerwave equipment, with shielding gas of 90% argon / 10% CO2, or 98% Argon / 2% O2.
Tim