Hard Or No Shift

OK it been a while since I got the engine in (M103). My car Doesnt shift into 4th gear. Or if it does I'm running at high RPM. While I'm going 80MPH the Tach read 5,500. I know these car run at high RPM but not this high. After driving for a while at 70 the trans doest pull no more, If I step at the gas harder the RPM just go up and the car slow down. The only way I keep the car from stranded on the road while driving is shift it to neutral and let the RPM go down to 2,500-3,000 RPM and step on the gas and shift it back to gear.