I just measured my Streetfire HEI (large cap) and AllStar HEI (small cap) from base of intake (where the clamp attaches) to the top of the coil. AllStar is about 1/8-1/4" shorter so you wouldn't gain enough on your '29. Both my engines are SBC. You can measure your existing HEI to determine overall height.
Here is a link to discussions about crab distributor caps:
SBC Crab style cap- Accel... Mallory...??? - THE H.A.M.B.
One writer suggests the cap is 4.76"-you might try to contact him. If that measurement is accurate you will "gain" about 2".
Of course, you will have to switch to an external coil with the new cap.
In addition, here is an earlier discussion of a similar problem with a '29.
Distributor cap-side plug wire terminals
Hope this helps.