I guess it depends. You either spend the money having a speedo cable made for your application or try to find an electronic speedometer to fit your dash. I'd go with the cable but that's just my opinion.

As for the gen/alt thing, I'm building a 50s style roadster and my 57 T-bird engine came with a generator. so it's still on there. I might change it to an alternator when it's time to wire it up. A generator won't put out enough juice to charge the battery at idle. An alternator does. Generators also need to have the brushes replaced more frequently than alternators. I converted the flathead in my 40 panel truck to an alternator and never had a problem in 12 years. If you're putting a T5 behind a flathead i'd guess originality isn't that important to you.