Thinking about replacing my leaf springs, and I need your advice.

My previous 225 Duster had 79 rear wheel HP, as actually measured on the chassis dyno. I didn't think that a chassis dyno could be calibrated to accurately measure such a low power output. My quarter mile ET at the drag strip would likely be about 23 - 25 seconds, with a trap speed of around 58 mph.
When you finish with your suspension upgrade, we can help you tweak that 225 until it is VERY happy! Cruise on over to the slant 6 area and learn from the guys who've done it!