From LUND:
You may not need the reducer, but we will need to know the measurement of the inside diameter where the MAF is located. This next part is important, you will want to shield this filter from any direct oncoming air. The sensor being located so close to the filter opening will cause turbulent MAF signal and will have fueling all over the place. The best we can do is shield the filter from direct outside air and hope it is not an issue when tuning.

have many of you used a heat shield to block any spurious air in the engine bay?

I believe Edwardb recommended this on his 20th anniv build
Spectre Performance 8130 Aluminum Air Filter Heat Shield

I am putting in the Breeze Radiator shroud which should help reduce some air into the engine bay, but I know air gets in the space - wondered if the $40 is needed?