I'm sorry but a lot of this problem does not quite make sense to me so I do look forward to pics...
FWIW unless you don't know a mountain bike from a road bike, if your frame is indeed a roadie (that wasn't respaced)...120mm rear tells me it is/was originally 5-speed. Are you able to tell us what brand/model frame you have?
And I am pretty sure that there were old UK bikes like Raleigh among others, that used wider bottom bracket shells than standard British/Italian sizes - like even up to 75-76mm wide.
But that should mean that as a result you need a longer-than-normal spindle to accommodate for the wider BB shell - not end up with your chainring too far out.
Maybe it's just me, but I won't use any kind of spacer-rigging on my drive train. My first thought is that if you are sure that your description of the situation is accurate you should carefully measure it all up and invest in a different bottom bracket with a shorter spindle and the right taper to solve your problem. But on 2nd thought...that crank
I once got talked into trying to help a buddy build his conversion from an unknown frame and we constantly ran into this kind of crap. Cured me of ever touching one of the damned things again...