if it's all stuff you already know how to do, just get the parts (freewheel and spacers), and just do it. don't tell 'em what you're doing with it...
my guess is, that if the axle is spaced for 135, there's gonna be one 15mm spacer, and one 5, or a 10mm, and a 5, in addition to the cones and locknuts, depending on how thick the locknuts are....you should just be able to get a bunch of 5mm spacers, and maybe some 2.5mm ones and set it up exactly how you need it. i can forsee some chainline issues with the hub though, since it's a 6-speed hub, on an axle spaced at 135, things might get a little funky, depending on how wide the actual hub body is, you may have to run a lot of spacers on the non-drive side and have a reverse dish to make the rim sit in the right spot and still get good chainline. but most 6 speed, threaded hubs are pretty wide.
as long as you respace the axle, properly, and redish the wheel, it'll work (something that you already know). just get the parts and stop being so chatty.
most shop mechanics just want to feel like they know more than you do. i'm pretty laid back with a customer, until they start trying to flex nuts, and i know they're not buying anything. then i shut 'em down...if someone's in there and they're trying to do something like what you are, i'm usually pretty helpful and very forthcoming with information. if they're all BLAH BLAH BLAH I KNOW MORE THAN YOU DO BECAUSE I HAVE DONE THIS ONCE BEFORE IN MY LIFE!! then i give them "the look," and tell them to come back when they screw it up beyond their capabilities.
...and nobody ever f*@#ing tips their bike mechanic.