No real problem using spacers on the rear. Your spacers should be about 3/4" thick, the wire wheel axle being 1 1/2" narrower than the disc wheel axle.
Just make sure the studs are long enough to secure the wheel properly with the spacers installed. You may find you have to replace them with longer ones.
Also, most disc and alloy wheels seat to the center hub of the axle, which better supports them. This is more critical with soft aluminum or alloy wheels, but stock MGB and Midget Rostyle wheels do not and I've worked with various wheel adaptations on other cars that could not seat to the hub, so I do not see this to be as crutial, but if in fabricating your spacers this could be accomplished, it might be something to consider.
Hope that helps you. Good luck.
Art