I have modified a bulgy polymer to work, but the weiger may give some trouble. The problem is the front strap/lug is too wide to fit in the mag well of the 84s. The weiger mags I've measured are all around .090-.095 wider up front than the norinco mags, and taking off enough material to match the factory mags won't leave much left. Of course, the magwell is cut a bit bigger for clearance - if you are willing to accept a tight fit, you can probably safely take enough off.
Of course, you could also always open the mag well up a bit, but it is always best practice to start hacking away on the cheapest part...
I can't say for certain whether or not the galil mags would work. However, given that norinco, bulgy polymer, and weiger mags have all been successfully used in Valmet conversions, and I've always heard it was bad juju to attempt to use galil mags in the valmet, that the galil mags would also have problems in the 84s. Would be nice if they did - not for 30 round mags, but for the relatively cheap 50 rounders.
Rocko