Where would I block it at? at the engine or on the line its self?Sounds like low pressure, should be between 56 and 62 psi, block off return line, retest...deadhead pressure should be above 70 psi. From what you described, my first two thoughts would be fuel pump or line restriction. Then possible leaky injector...but I would get the pressure where it belongs before I worried about injectors. Blocking the return takes the fpr out of the equation.
Warning... Above 75psi is rough on the regulators...so be ready to kill pump power or release return line...it will build quick if it does actually build. (I'm expecting, it doesn't though)