I would get a front pump out of one of your cores. Making sure it’s 1971-1977 stator.
Take it apart, clean it, re assemble it. Then use that in your transmission. I would also tighten up the end play to .020-.030 assuming you have thrust washers in your geartrain and not Torrington bearings.
I am by no means someone who claims to know it all, because I do not. There may be others who disagree, which is fine, but if it were a transmission that I was building for someone who is experiencing the same issues you are, that is how I would approach the situation.