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15865791 is the part number. I will have a look and compare what I have thank you for that

According to https://www.ebay.com.au/itm/166239746265 the interchange list for the part number is:

FITMENT:
2010-2015 Allure 2.4L, 3.0L
2010-2016 LaCrosse 2.4L, 3.0L
2006-2007 Rainier 4.2L
2011-2016 Regal 2.0L, 2.4L
2016-2019 ATS 3.6L
2016-2019 CT6 3.0L
2014-2019 CTS 3.6L
2007-2011 SRX 3.0L,4.6L
2006-2010 STS 4.4L,4.6L
2006-2009 XLR 4.4L
2014-2017 XTS 3.6L
2010-2015 Camaro 6.2L,7.0L
2005-2010 Cobalt 2.0L, 2.2L, 2.4L
2007-2012 Colorado 2.9L, 3.7L,5.3L
2006-2013 Corvette 6.2L,7.0L
2010-2011 Equinox 2.4L, 3.0L
2006-2011 HHR 2.0L, 2.2L, 2.2L, 2.4L
2014-2016 Impala 2.4L, 3.5L, 3.5L, 3.6L, 3.9L
2008-2014 Malibu 2.4L
2013-2015 Spark 1.2L
2006-2009 Trailblazer 4.2L
2007-2012 Canyon 2.9L, 3.7L,5.3L
2006-2009 Envoy 4.2L
2010-2011 Terrain 2.4L, 3.0L
2007-2010 H3 3.7L,5.3L
2009-2010 H3T 3.7L,5.3L
2007-2010 G5 2.2L, 2.4L
2006-2010 G6 2.4L
2005-2006 Pursuit 2.0L, 2.2L, 2.4L
2006-2010 Solstice 2.0L, 2.4L
2008-2009 Aura 2.4L
2005-2007 Ion 2.2L, 2.4L
2007-2010 Sky 2.0L, 2.4L
2007-2010 Vue 2.4L

Then I guess you need measurements of the intake pipe diameter, and source your data from the closest sized intake pipe you can find. You mentioned your pipe was 3" (thats what she said), are you staying with that?

how would i find which has the a 3" intake?

LOL Yes thats what she said and yes thats what its staying as everything is 3" ie cooler inlet and outlet throttle body etc

... and that is the million $ question. I don't have the answer. Probably just hours spent guessing and searching the internet and thinking about search terms to use to find some reference on some forum somewhere that gives you an idea. I guess similar engine size to what you're running seems like a sensible place to start, but from there - I don't know. The internet can be a time killer. But since most people seem OK to tune with the injection multiplier and say don't touch the MAF scalers, I think it wont matter so much if you don't have MAF tables perfect. In my office job I'd attack a problem like this by putting a time box around how long i'd spend doing the research then once that time is spent if I think I had reasonable data I'd roll with that and start tuning. Your call though on when to move from getting the MAF right to tuning as if you change the MAF table later it's going to throw any injection adders and timing table data out so you'll need enough confidence that it's in the ballpark and going to be useable.

I think that is where I have ended up as everything is either 2.75 or 3.25 as closest but then engine sizes are either way smaller or way bigger and whos to know the air draw they will have either. So I have made a spread sheet that is bang on down long then tapper up to the 16khz as all the data I can get only goes to 12khz

yeah don't worry about the above 12khz, I'll be surprised if you see it, and if you do, then you probably need a wider diameter intake and that'll bring it back down again.

Typically seen around max 10,500 on a few cars I've played with to give you a guide. Highest was 550rwkw, around 650g/s of air at around the 10kHz mark. That was LSA MAF sensor in a Harrop 4" tube.

Same MAF in different application[intake length,diameter, filter position, bends, throttle, intake manifold] use quiet different curve. So getting one close from stock application will give you some start point but not near close where you want it.

Do you guys also change the min maf frequency in the bin? factory is set to 1890hz but the ls7 maf seems to start at 1000hz

I would just do an Idle Log and see what hz the maf is reading at the min desired idle rpm. Then set your min maf frequency 2-300hz below idle frequency. That's if the motor is already run in and not fresh and idle afr is reasonable.