I think I have come to the conclusion that in order to get away with any of the manifolds I have on hand, I will have to make too much of a compromise. The indy manifold will just not allow the room needed for radiator and cooling fans That's going to be going up for sale.
Looking at Lengel's and Carroll's cars, space was made available by using a shorter Scirocco-style radiator that would tuck under the plenum. I wanted to do the same, but all of my manifolds hang down too low to allow that.
In discussion with DJ, he has had success with a 1st gen Neon radiator. That seemed like it might have been the solution. With the vertical flow, two small fans could be placed on the lower edge of the rad and allow room for the plenum up top. But when test fitting the rad, the inlet nipple would be in contact with the large plenum manifold I have.
So it looks like a custom solution will be in order.