Large port manifold to small port head might cause gas to drop out of the mixture as it hits the "wall" surrounding the small port. Causing all sorts of weird issues that'll have you thinking the carb needs more tuning.
I could be wrong of course
It wasn't as bad a deal as some make it out to be, Ford did this on a couple of factory combos, on the 429's Seen this on a factory jet boat engine too with an SCJ intake on an otherwise stock 460.
So far the intake/head combo is running like a champ. Pulls hard through all gears at the revs I've been running so far. Idle is smooth. Vacuum is steady.
When I put it together I reminded myself my aim was a low rpm engine to run in a truck that weighs 4900lbs. I figured the port mismatch would likely cause problems at higher rpms, if any problems were to arise. And I doubt I'll spin this thing 5 grand.
If it ever becomes and issue I'll sell that intake, cause I guess it could be kind of rare, and buy a performer rpm or similar.