289 PIN Drive Backpace Question

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Hey,

I am going to use Vintage Wheels 15" FIA wheels with the pin drive adapters etc..
Vintage Wheels has 15" x 7.5" and 15" x 9.5" with 3.7" backspace on all wheels.
My 289 front suspension is pin drive width, ready for the standard 3.7" backspacing using SN95 spindles and FFR pin drive width control arms.
I am running the newer IRS so I will need Vintage Wheels to customize the rear wheel backspacing, which they will do.

FF 15" wheels have a 5.48" backspace.
FF 17" x 9" wheels have a 6.00" backspace. (Which I was sold as a set of 4 as standard for the 289)

I'm not sure why the different diameter wheel would have a different backspacing from each other so I'm not sure what backspacing I need.
I was going to ask Vintage Wheels to make the 15" x 9.5" 3.7" backspace, match the FF 17" x 9" wheels backspace of 6.00".
I'm looking for any thoughts on why that might not be correct before I drop about $4,000 on these.

I've seen pics of other FF 289's with pin drive wheels but haven't been able to make contact with any owners yet to talk about it.
They may be using the older Thunderbird IRS. Not sure.

Thank you!

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I'm going with 6" backspace on the rear.