Currently we use lead free solder in a high temp high vibration environment, the lead based solder is not as versatile, so there are some advantages. Populating on polyamide substrate requires us to use a very high temp reflow, approx 265'C. This does cause some drop out in weaker components but...
Scrag,
You have not made it clear where your control voltage is from or at what level it would be. Your simlest solution could be to use a solid state relay, these are available to handle large currents (50 amp common) and work at various voltage levels including 240V.
Although Ling may not have the diagrams for it, LDS, Ling Dynamic Systems split and became a separate company with the same equipment as such. They may be able to supply a set of archive drawings.