Searched Google for "75mm throttle body", there are several even up to 80 & 90 mm. most are under $250. The one made by Accufab Racing is really nice, appears well machined and even polished. These all have ball bearings and o-ring seals. Most of the 75mm ones are around $200. Seems most are designed for late mustang 5.0L . Car appears to have emphysema. Any way, this seems more reasonable than building one unless there is something similar in a junk yard.
The idle and taxi speed issue could be handled with some form of progressive linkeage. The cable drive ones had a nautilus type cam that began with very slow movement off idle. This will not work with a HD push-pull cable.
...Chrissi

Chuck, here is some businesss if you want it.
Get your machine shop guy to wack out a bunch of these throttle bodies.

Paul Lamar

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