I work with a similar "build to print" operation. Unfortunately using fabrication and painting from outside sources means difficult lead time determinations. Unless you are a large percentage of their business, they will fit you next in line. This is where the little recognized art of supplier relationship is most effective. Poor relationships mean you get it as soon as they can fit it in at best.
You must look at your historical data and extrapolate your educated estimates from that, meaning you will be able to shorten them or extend them depending on the current business situation at the source. You must set your leadtimes to the best available data and review on a periodic basis accounting for fluctuations. For the past year lead times have been reduced because everyone was hungry for work. Now lead times are starting to expand as the sources get busy.