1) I recently replaced a failed D-jet pump with a generic MSD brand pump from Summit Racing, cost around $90 including mounts. Only minor adapting was required. On a '70 or '71 with a three-port pump, you'd have to do a bit more plumbing, but it's still simple enough.

2) Half the D-jet problems I work on are just a matter of tuning what some previous person has adjusted incorrectly. It is capable of running extremely well.

3) With some D-jet parts starting to become unavailable or ridiculously expensive, MS is indeed a good option... or the Simple Digital Systems injection if you're not up for doing a lot of soldering or cable harnessing yourself.

Note that a Weber downdraft doesn't exactly come tuned for your motor out of the box. It's not all that simple to get them set up right, and even then performance is limited.