Ya know, I've never heard of such a kit either but I've owned three loco kits produced by KTM that fit this description. All were sold by PSC or Westside. Actually one was sold by PSC but in a red Westside box. It was an SP TW-8. Hundreds of pieces. Basic mechanism done, boiler roled with running boards attached, cab and tender shell formed.
The other two kits were SP Cab Forwards (AC5 & 6 I believe) These came in PSC (orange) boxes. Even more pieces but same scenario for pre-assembly.
I guess what I'm trying to say is, if PSC imported a Big Boy around the same time it's possible they produced such a kit.
None of the kits really had much in the way of instructions.
If it were me I'd try to borrow a similar model that's been factory assembled and use it for reference.
Butch,
Any knowledge of PSC importing BBs?
Jay