I suspect a 7 speed freewheel will fit. If not, maybe try just a 1mm spacer on the DS--being 1mm out of dish probably is imperceptible.
As I mentioned, 7 speed and 8 speed spacing is .2mm different, but being 1.4mm off in total means that if you center it over the middle of the cassette it is only .7mm off at most, which is much less than the play in the guide pulley, and in my experience works more than acceptably.