The FBW 50mm and 58 mm HD throttle bodies are the same bolt pattern and ID at the mounting face. Bolt it on and ride.
Please post your dyno chart!
SG
Sorry, but that's not accurate. (Unless I misunderstood your statement)
While the bolt pattern is the same, the ID of the throat of the TB inlet and the needed AC backing plate is demonstrably larger on the 58mmTB.
HD sells a different backing plate for the Ventilator (for example)
P/N 29763-09 - Backplate assembly, 58 mm throat size (for kits 29490-09 and 29915-09) For the stock 50mm bore TB
P/N 29400038 - Backplate assembly, 64 mm throat size opening (for kit 29400039) For the SE 58mm bore TB
or you can do this