BG is listing these as having 1 3/8" primary bores, which would basically be OEM sizes for smallblocks.
From http://ramchargercentral.com/engine/thermoquad-guide/:

Quote:

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The Thermo-Quad was available with two primary throttle bore sizes, 1-3/8"
and 1-1/2". Flow ratings (CFM) vary depending on the source, but
the TQs with the 1-3/8" bores are listed as 750-800 CFM and those with the
1-1/2" primary throttle bores are rated at 800-850 CFM. All TQs have the
2-1/4" diameter secondary throttle plates. The primary bore size depended
on application. In general, all 78 and later 318s and 360s and all 340s had
the smaller bore. Earlier 360s varied depending on application, most 400s
and all 440s had the large bore. The 9000 series have the small primary bore
and were rated by Carter at 800 cfm. Later TQs (ie, Lean Burn and ESA
applications, feedback systems) are quoted with less flow ratings, but
this is due to the control of the carburetion system, not the inherent
flow capability of the basic carburetor. The internal metering is set for
leaner running conditions for Lean Burn. Either bore size can be tuned
to run well on most engine combinations. The smaller bore offers a slight
increase throttle response but less overall flow. The different bore sizing,
ie. speadbore, is an aspect that can lead to increased fuel economy while
delivering similar wide open throttle (WOT) performance to an equivalent
standard bore configuration. The adjustability of the TQ and the
spreadbore configuration allows the use of a large CFM carburetor on
a small displacement engine.

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I'm curious how well this would work on a '65 413HP? I'm not sure I want to be a guinea pig for it, although if it's a bad fit I believe Summit would take it back with no questions.

And I wonder if an OEM 1 1/2" baseplate could be installed, would it fit and would it work well???