I also have been working on getting more power from my 401 and have had the same problems. The higher flow airbox mods give you more mid to top end power but will hurt the bottom end a bit. You will need to chaing the gearing. This helps but does not fully fix the problem. I have been doing some testing on 17 390 duke and the 401 with different exhaust a found that most of them have to much volume and kill low end power a bit. Most mid pipes are out of 2” tubing an sould be a bit smaller to get some more back pressure. I know this now because i put a reducer sleeve in side my mid pipe take up some volume and it made a big difference in the low end. Its not perfect but way better all the way through the power curve. I think the next step for me is to build my own exhaust.