turn off the m42 at the shirley juction and follow the a34 into brum, much better option
Depending on what time you will be traveling Sonic??
if it's an evening show don't go near the Shirley Junction. Do as you have said. Get off M42 at J3 and follow the road straight up to the Bristol Rd (can't remember rd number) then left towards five ways roundabout and right down broad street. Turn left at the First signalised junction and you can't miss it. Real pain in the arss to get out of in the evening though. Well depends who your seeing.
Get there early enough walk round to Brindley Place for a beer. Nice area