Bruce,
You can improve your airflow at speed by fabricating small (say 2" x 3") air passages in your fan shroud. Place them close to each corner and attach a flap over each of rubber or some other pliable material. At speed the flap opens by air pressure against the flap and allows more air to flow through. At idle or slow speed when your fan is on the flap closes and causes all the air to be pulled by the fan. Hope that makes sense.
I had a similar set up on my Lone Star as you have fabricated and my fan shroud had those little flaps. Here's at picture but you really can't see the flaps.
After doing the inlet shroud and fan shroud I never had any heating problems and ran under drive pulleys.
Nice work with the aluminum!
