Aside from which block/head it is, if you have a stuck open valve there's a good chance the problem is down inside the block with the lifters.
Try to extract the lifters from the top and see if one matching the stuck valve is also stuck at the top of travel. If they don't lift out easilly, they may have caused the problem you have with the valves. If this is the case, you may have to pull the motor to get the lifters out from the bottom. Check the cam for damage also.
I would dis-assemble the head and look for bent valve stems and broken springs.

If I were you, I would get a good B20B head and stick with stock B20 components for lifters and push rods, not mix the B18 head and B20 block.

A B20B head on the B20E block is a great performance combination for a carb'ed engine.
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'89 245 sportwagon, destroyed by hit & run driver, RIP. '04 V70 2.5 T Sportwagon, 12k mi and '91 245 5-speed, 209k mi, replaced the '89