What motor?

[lightning bug]

Probably not the best choice. Aside from all of the work involved (I've never done this conversion, but can only imagine) weight is definitely a factor. A T4 weighs approx. 300 lbs. Depending on which 6 cyl. you choose, you will add up to 200+/- pounds to the rear. For example:

2.4L........400lbs
2.7L........440lbs
3.0L........460lbs
3.2L........480lbs

These are DRY weights and without transmissions, (you will need a Porsche trans which are heavier).

A'74 911 weighs 2500 lbs., 2.7L with advertised 180 HP in the 911S.
Your bug doesn't weigh more than 2000 lbs. You can easily get 150 HP out of a T4 or T1 for that matter. So for 30 more HP and 140+ lbs. more rear weight, plus the fact of all of that work. You be the judge.