You say B20 and mention extractor clearance. Are you running the alternator on the drivers side?
I assume you know this, though if there are two large threaded holes on the passenger side of the timing case you can put an alternator there, as big as you like. B20s often have this. If this is an option for you, call Voldat or Berrys and get an alternator and the lower and top brackets, bolts, new belt, re run wires, done.
If you are drivers side, you probably have the iPd or Skandix aftermarket mount onto the block. My B18 will foul anything that is not an SEK on the chassis or steering box (in an 1800S) as an SEK Marchal alternator is smaller than a Bosch. I went with the above mentioned 55A SEK from a late B20 and found a solid state external reg that I mounted to the rear of the alternator. It is absolutely an external reg btw. Pretty sure the same unit was on the old 140 before it was sold. I know I bought two regs back then. The trouble will be finding a 55A alternator. They are distinctive as they have a large red heat sync on the back, the 35A units from earlier have a much smaller heat sync.
I can take a pic in a day or so when next at the workshop if you like. The reg was a bugger to find too, took literally 10 or 15 night time hours to find a unit that would work. I think it came from Hungary, will look for details if you like. And it works a treat.
Depending on the chassis, the VN mod may not fit, i sold a VN modded alternator a few weeks back to a guy doing exactly what you are planning for a 160. Maybe scroll down and have a search for that as a reference.
There is also the later B20 Bosch alternator. The simplest of the lot. Factory for later engines, it was on the passenger side, it is bigger than the SEV Marchal. Someone (Angus maybe) had a few old engines, he may have some old alternators to test clearance with. Just check the alternator is clear of the steering box and chassis. I do recall an after market mount that used the fuel pump bolt holes to put an alternator on the passenger side, though it looked very flimsey.
Also check your wiring, I found a butchered up Bosch alternator on my 1800 rated at 65 amps and the factory 40A cable. It is now a more appropriate alternator and solid wiring (for the alternator anyway, the rest is yet to be cleaned up).
