There's too many variables to tell where the optimum angle comes into play without the vehicle's final assembly including the dearching of the springs if you go that route...Drilling holes now will be a guess at best and you'll probably end up making more when it's all said and done...
I'd either install the shackles in the factory frame holes until the final assembly is done and you know what you've got to do...Or{if they don't fit} don't install the shackles, use a wood spacer between the frame and the rear spring eye and let the spring slide on the spacer until you're done assembling the car and have added all the weight..

Here's a diagram from the MP chasis manual...