I did the same upgrade on my'03 and I had the same leaking issue when I first installed it, and it was because of the Teflon tape. I reinstalled it with a liquid thread sealant and it worked much better, no problems after that. One of the best peace of mind upgrades I ever did.
2003 Pitch Black SVTF, K&N Drop in, CFM dog bone. 155,000 miles. **SOLD**
2001 Sentra SE 2.0 w/JWT Pop Charger. RETIRED with 511,000miles.
2008 Setra SE-R Spec-V 187whp. 167 lb. ft.