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I doubt there's 1 cm diameter difference between the two. From your wheel measurement, it looks like 71HTA radius is 2 or 3 mm, which would add up to about 0.5cm diameter. Either way, this could be why your hotside isn't fitting. Measure the inner diameter in your hotside and it's probably smaller than the 21 cm you measured.
I double-checked the circumference of both and it was 20cm for the factory turbo and 21cm for the 71HTA.
The turbine housing came in at right around 20cm so this definitely appears to be the issue.
I'm happy I wasn't screwing up something simple. I'll reach out to FP and have no doubt they'll take care of me.