From what I remember, Spot Welders are Resistance Welders...is this correct?
If yes, then the operational characteristics of a typical unit will have a very high inrush load current - which is sustained for a long period of time, with the load current becoming slightly reduced as the contacting points increase in heat.
Voltage drop - and system loading, will always be problem factors with Resistance Welding Equipment.
If possible, place them as near to the Service Equipment as possible. This may assist with the Voltage Drop problem for the Resistance Welders.
The extended surges of these welders will affect the remainder of equipment on your Load side of the KWh Meter - AND others on the Secondary side _ AND the Primary side (when the Primary Feeders are long, loaded and small in size).
Also, the KVA size of the Power Company's Transformer (or network) will totally affect + be affected by these welders - and overall, the system's Voltage.
Many PoCo's (Power Companies / AKA "Utility Companies") have certain demand calcs for "Welder Service" (found in their "Description of Service Manuals" or "Electrical Service Requirements Manuals").
Typically, Motor Generator type Welders & Transformer Arc type Welders are in a "Class" where ratings are taken at 100% nameplate values; whereas Resistance Welders are classed in multiple factors - depending on the exact task type and overall "Tweeks" of the Welders.
As to the existing setup - where the large capacity (nameplate 400A, 380 volt) spot welders are causing lighting etc to dim:
[*] What is the nominal load (KVA / Amperes, etc.) at the time of dimming lamps?
[*] What type of Lamps are dimming? HID, Fluorescent, Incandescent, All,
[*] What is the intensity of Lamp Dimming + time duration?
[*] What is the KVA rating of the PoCo Transformer feeding your Service?
[*] What is on the same feeder(what other equipment - if any) which the Spot Welders are connected to?
[*] Does this Lamp dimming problem occur during certain time periods, or is it something that would happen at any time - day or night - weekday or weekend?
I am thinking maybe there's a System Capacity issue.
If the Circuitry feeding the Welder has Capacity + compensated for long distance run, then the Voltage Drop at the Welder should be minor - and the entire system would react accordingly to the sudden surge + extended load.
Incandescent type Lighting will show the most drammatic effects. Fluorescent Lamps will react as well, but not as much as Incandescent Lamps.
HID Lamps may diminish in intensity, or may completely restart - all depending on the type of Ballast used and the Lamp Characteristics (+ age of Lamps too!).
Insufficient Circuit size to the Welder may result in less effects to Lighting (as would be seen with proper size circuitry), and would place the Votage Drop problems mostly on the Welder - at the Welded Work it's self.
Take a look at these items I have listed here, then apply them to the Project.
Let us know the results!