0.005" is not enough to make a difference in the timing chain slack.
Your stock 78' L28 is around 8.3:1 CR. Putting the P90 on there with the stock dished pistons will drop the CR to around 7.4:1. So if plan on using the P90, you need to shave it or you will make less power than the N47 you have on there now.
What are you HP goals? I realize you just got the P90, but the N47 will do what you need without shimming or shaving. The problem is that whether you shim and shave the P90, or just keep the N47, your CR will be too low to make enough power to make the work worthwhile. To make decent power on a street NA L28, your CR should be in the 9-10:1 range. The best way to do this is flat top pistons and an N-series or shaven P-series head. If you don't want to mess with the short block, do some work on the N47 and use a Felpro gasket to get a little more CR.