I made myself a dish gauge with a long piece of 1/2" wood dowel and three long wood screws. Two screws were placed into the dowel far enough apart that the heads line up with the diameter of the rim. The third screw was inserted all the way through the dowel from the other side and can be turned in or out to interact with the axle locknuts. (this will make more sense if you check out a picture of a dish gauge)

You can use your rear triangle, brake pads, and the homemade dish gauge to get a pretty well set up wheel.

Your frame spacing isn't too big of a deal either. Check out cold-setting a frame on Sheldon's site.

I agree with the other folks that you may end up putting a lot more money into this than is sensible. The point of a conversion is to do it dirt cheap. You can respace the wheel and throw on a $10 freewheel and be set for awhile while keeping your eye out for a decent ride with aluminum wheels.

You could potentially replace just the rims (~$10 apiece for 27" at Nashbar) but then you're still out $40 or so for spokes. Then again, if you did that, probably should drop $50 for a Formula flip/flop rear hub. With that in mind, $150 for a garage-sale bike with alloy wheels isn't looking too bad.