Life after PI heads

I have a 1998 Mustang GT with manual Tremec 45 and Romeo NPI engine, that I fitted with PI heads right before being deployed. I ran the engine for a bit and did not get as much power gain as expected, and was running on the lean side. I bought a SCT programmer and re-mapped the ECU with the mapping from SCT according to my mods (PI heads, Bosch platinum +4 sparkplugs and K&N filter in regular canister). After programming it, I noticed that it knocked on hard acceleration at lower rpms; from the original programming I delayed the advance 5 degrees under 3000 rpms and 3 degrees after 3000 rpms; I also enriched the air/gas mix an additional 4%. That seemed to cure the knocking and feels that it gained a few ponies.
I know I can gain more power in a conventional way (no nitro and normal aspired) so I bought on eBay a set of Stage 2 Crower Cams. part # 62802-2 with the followin specs:
Intake: 254 Lobe 258 and 268 Valve 272. Exhaust: 208 Lobe 212 and 224 Valve 228. Lift: .559 / .570
Q: What else do I need to do besides the cams without spending too much?

Thanks a lot. When I said lifters I meant the followers, and they seem to be in good shape.
Unfortunately I did not do dyno prior to the change into PI heads and cams (Winsor), since I did it incidental to having to remove the engine due to an exhaust manifold bolt snapping off: Since I had the engine out, and someone came around with used PI heads I bought them and had them reseated and tested on a good head shop (Dover Heads in Jacksonville).
I will go ahead and get springs, reseat the valves and ask for a quote on stage 1 work. But will change the springs and cams for sure, and re-tune the engine; this time using the oxygen sensor (need to buy) for the SCT programmer.
Thanks.
Any other suggestion for additional good bang-for-the-buck mods?

Thanks. Definitely getting smarter on this. I will install the cams and springs and re-tune it with dyno.
Any significant added value with porting, and if so, what stage you recommend?

Thanks Ryan. I guess it looks more and more like I will have to go for stage 2 porting as well.