I did this to my 67 220 when my B18B went kaput. I put in a 1971 B20E using the soft plugs that have been mentioned previously.

Regarding the breather box, the B18 definitely will not fit. The other critical bit is that the B20's have a threaded PCV valve that goes on the intake manifold, not down with the breather box.

A 1971 B20E is considered the best. A high compression 130hp engine. Earlier models had fewer HP, later models (-f versions) have smog setups and scaled back power. Any would work. I'd guess you'd be better off getting a B20B which was a carbed version.

I've done an alternator conversion to both my cars and have never regretted it. If you are going to swap engines, you might as well get one.

Notably, my 1971 B20E had a different thread pitch for the alternator bolt and this caused me some grief. Not metric, just coarse vs fine with the same size bolt.