Why not make your y pipe 4"? The 3 inch portion won't kill you (see my results with a single 3"), but will be more restriction than the entire rest of the exhaust.

And a 4" intake should feed it fine, just be sure to scale the MAF for the tube size where the sensor is.

 

what injectors are you running? do you know what duty cycle there at?

 

I don't think he will be over 20lbs with his set up. I'd bet more around 15-16lbs. I had a 2.5" and stock crank pulley on mine and was a little over 13lbs. Stock manifolds, y pipe, cats and 2.75" exhaust/muffler. His will flow better than mine and his cam is slightly bigger and on a bit tighter lsa. Gonna loose some boost there too. Stock pulleys on mine made 8.5lbs and I'm running a 2.85" now and am right at 10lbs. If it were me I would start with the overdrive crank pulley and stock upper to see where you are at. Then pulley down if you wish and you have an idea on where you will be. These are just my opinions though, based on my lsa blower experience so far.

 

The way I figured it put him at about 19 if there isn't belt slip, which there almost certainly would be on a 6 rib and that small of pulley and that drops back to about 16 without the overdrive crank.

I am running 6.2 flex fuel injectors but don't know the duty cycle. I have to be getting close to maxing them out, but if I pulley down and run out, I'll probably swap to a id850 or id1000s.