All perfect and Important Points Given by Matt and Ken here and I only say cause I have done the 6.1 install on my truck twice. Some additional Points. Retainers I re-used mine, Keep the buckets, I removed mine but only because of my T-Handled long spring Tool I used. The Dragon-Accord Home made Tool I aquired later and with it you will not have to remove the brake booster or buckets if leaving the heads on. I have personnally found a broken 5.7 bucket tab on my car when swapping springs but given the machined circle the broken piece was just sitting underneath and I grabbed it with a magnet.
The 6.1 oil Pump pick-up tube on the trucks is long, and what I did was just lower the pan enough to get my hand in there to remove the bolt midway in that attaches it to the block (small ratchet wrench) once undone you can undue the second bolt that threads into/under the pump.
Couple other parts to consider and are cheap.
-Timing Chain and/or at a minimum new Chain Tensioner Plate and black sliders ($16)
-Valve seals if doing springs (I never thought of this....before...Damn) just slide on new ones when springs are changed ($25 I think/felpro set).
- With the 6.1 Cam I would consider doing headers to try and regain the low end torque lost.
-Inspect the damper Rubber material for deterioration and rips and or if its starting to squeeze out where bonded together, or spend $300 on an SLP or like I did just put in a new OEM Mopar on at about $65.00 (Good insurance).
Good Luck and glad to see some 03 action on here. Danny
Edit: I am removing my ported TB this week and may offer it for sale as per the Forum Rules in the near future on the For Sale thread.