Glen 400 psi sounds a little low how ever it does make some sense the pressure would have to be lower than the injectors cracking pressure! Tech ,That said the guy with the failed piston likely had a bad injector to add to the advanced timing issue, we once had an engine come to the shop pump installed by the owner and all 8 pistons were burned up by a ill timed pump! On The Timing subject , I have used the remove the delivery valve method with a little different twist ,for years and it never failed me once !The method I was taught as follows ! remove #1 Delivery spring and needle valve reassemble hold down nut to pump , remove the inlet fuel line to pump make up a #6 one ended fuel line to fit on to the inlet Set the engine on the suggested timing mark Loosen the pump slot bolts leaving the top one snug on install loosen that snugged bolt after the pump is on and simply blow thru the hose you installed on the inlet and turn the pump till you can't blow or just see the tiniest amount of bubbles at the #1 line connection on the pump and simply lock it off at that point! Then test your set by turning the engine back a 1/4 turn and move back to the prescribed timing mark! 12 -24 degs depending on the model and year! It's fun to see everyone's different methods of getting it done !😁
The above method is best used when installing a new or reman pump as everything is off anyway and can be done with out getting fuel allover the place or just hooking up minimal lines!