The standard Spitting Cobra is a fearsome fire-support design used by Southern forces. It's default armament is intended to allow the unit to strike at targets at a variety of ranges. However, variants of the Spitting Cobra exist for a range of specialized tasks including anti-aircraft fire, engineering support and heavy assault duties.
One of the most common variants, developed during the War of the Alliance, is the Support Cobra. This variant replaces the Spitting Cobra's rocket pods, Light Guided Mortar, and heavy Autocannon with a Very Light Field Gun and Very Heavy Autocannon.
In Heavy Gear Blitz! a Spitting Cobra can take swaps to these two powerful weapons individually. The Southern Vehicles Compendium 1 mentions this as a common option: The Very Light Field Gun and Very Heavy Autocannon are both ammunition-hungry an expensive options for a military force to field, so many forces take one or the other to conserve resources. THe primary requirement to be considered a Support Cobra is the rear-mounted Very Light Field Gun.
The Spitting Cobra is sold with the parts to mount this weapon, btu the assembly can be a bit confusing at first. To assembly this weapon option, the images at right may be helpful. You will need a Spitting Cobra V-Engine, the Very Light Field Gun, and the small Mounting Bracket piece.
The Mounting Bracket attaches to the V-Engine by being glued to the bottom of the engine. Make sure the bottom of the V-Engine is smooth and the small ledge extending off the mount will be flush with the bottom of the engine. The Mounting Bracket has a small triangular support on the left side (looking at the V-Engine from behind) with the small plate on the bracket marked with an X off to the right. The triangular support can be flush to the upright cylinder on the V-Engine. The arms that hold the gun extend down.
The actual Very Light Field Gun fits between the arms on the Mounting Bracket. It is scukoted with the weapon mount and should slide between the Mounting Bracket arms without issue. Make sure the gun is at an angle you like and does not interfere with other weapons and glue.
The Support Cobra is a great option and this guide show help assemble it.