Don't use angle iron on the runners. They will be amazingly strong when welded to a plenum. Plus angle iron will just look cah cah.
Do you have a router? Make a hardwood hammerform die with a routed quarter round over bit to shape the entryway. If you want to get really fancy make the runners protrude into the plenum by the distance of the radius.
The plenum should extand past the end runners a bit. Try to elevate the runners so the airflow does not have to make a right angle turn in the plenum.
This one required five different hammerforms to get the shape I desired:
FORD 300 inline six - THE BEST KEPT SECRET IN DRAG RACING!
