IMHO, I don't believe the Darrin needs disc brakes. I drove my Darrin for three years, on the freeways at speed, etc. and did not have a problem stopping even at those speeds. The 11" drum brakes provide more than enough braking for a car that is approx. 2000 lbs. I don't know about the Henry J, since I do not own one.
I was going to upgrade my brake system with my restoration and decided against it. I will use Silicone brake fluid (Dot 5) as was done in my Traveler when it was restored approx. 30 years ago by Harvey Hedge****** and is still going strong and working fine.