This is only for 328, and 323’s. 325’s already have the larger intake on them.
Increased high end torque, and extended torque range
Some loss of low end torque without a tune
Acquire an M50 manifold; junk yard, eBay, and Craigslist are all good options. Make sure it is an OBD1 manifold.
**The easiest way to be sure its an OBD1 manifold is the Pinwheel on the front of manifold and notice the angle of the bottom. All runners appear to be connected as well. Also, you can look for part number 11611735727, manifold part numbers ending in ...728 and ...730 SHOULD be good as well. P/N 1161720612 is good too but may require minor modification.**
Install it.... This isn't HARD, just time consuming and you need to be certain to properly route all the vacuum connections. To make things easy, there are a few Supporting Vendors that sell an Install Kit. It's not required, but it is certainly more convenient and less work is involved. If you go out on your own, you can get the parts from the home improvement store. The hardware store route is much cheaper, as the kit is a couple of hundred dollars.
This is not a difficult swap. Everything is easy to access, and relatively easy to remove and reinstall.