Lots of good advise here. I can tell you that I ran a very mild 383 for a few years. I used the same orange ECU with a stock dist I recurved and in fact that same dist is in the 400 in my sons Dart right now. I installed the MR Gasket spring kit and set the total at 38 all in by 2000 rpm. I let the initial timing fall where it did as long as it had full timing by 38 I was happy. The initial was around 18 to 22 when I actually looked at it. It had a stock bottom end including stock pistons. The 452 heads had some mild pocket porting and were cut for 9.5 comp. I had the MP .484 cam in it and used the Eddy RPM intake. Had a 3000 tight TA converter and 3.91's all in a 72 Dart. It had great bottom end as it would fry the street tires even if you nailed it from a 10 to 15 mph roll and it ran as fast as 12.30's @ 110 at the track.
I would take some of the advise these guys have given you and get the initial timing up more and definetly 3.91 gears will help. Good luck , Ron