Interesting... I’ve dealt with 2 of them. One at WesTech (we ran it on the 528 I had built years ago) and a friend has one we ran on his engine. The baseplate is just a tad bit larger and they have “J” slots in them to fit either bolt pattern.
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In the Overview description "features"
Elongated (dual pattern) mounting holes allow fitment on intakes with square flange (Holley 4150™ style) or large flange (Holley Dominator® style)
The BLP base plates have the correct dimensions in the length and width and location of the T-Slot and position of the throttle shaft and so butterfly location vs. the T-slot to provide correct calibration of the part-throttle behavior of the fuel discharge rate from the slot as throttle position is changed.
CliffsNotes: Carb has a good chance to work like it should with the BLP base plate.
I just spoke with FST this AM on one of their carburetors, he did in fact say that most of their line up except for the Dominators use a dual pattern base plate, may be ordering one this week. Might be able to buy the base plate cheaper from them.
The mounting bolt hole arrangement isn't what is important about a 4150 base plate. You would be better off with a base plate with a broken corner that has to be clamped down with a big flat washer if it has the correct T-slot than the prettiest shiny anodized billet chinaderp beauty with the slot too wide and too long, which it seems most of them have. If the slot is too big, too long or too wide, it will be difficult, near impossible, to tune the level road steady speed cruising so it won't be slobbering pig stinking plug fouling rich. By all means get the cheapest one you can find.
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