Timing retard under boost

I'm building a single turbo setup for my 5.9 build and am exploring options for timing control. I know most folks use the MSD 6530 unit but are there cheaper alternatives? My build will be an E85 blow thru, non intercooler and I don't intend on going past 8-10psi. I currently run one of the HEI ready-to-run distros and I love it. Is there anything on the market I could plug into it and map out timing retard under boost?