the biggest reason I can see not to use 3 carbs on these trucks is the rear one runs into the fire wall. At least that is the conclusion I came up with. I never actually bolted the intake on, because it seemed really obvious it wasn't going to clear, but then again that was 20 years ago, and I may not be remembering correctly. This is the same problem I am having with my fuel injection unit, the rear tube runs into the firewall.
I think that might be the most valid reason that they aren't used on trucks. I should have remembered because I originally considered an early corvette setup (side draft) but it wouldn't clear the steering column.
But it did bring back memories of those great SU carbs I used to work on. Might be the best 1bbl carb ever built.
Thanks for all the feedback.
