transmission not correct

I got a junkyard tranny ,from a good source, it came out of the same year and model 88 E350 C-6 but i cant get over 60 mph. can anyone help.

We need more information as to exactly what is happening. The transmission isn't shifting into third gear or the engine doesn't have any power, or the transmission slips....what?

It's not the engine, it is new. I know for sure it is shifting into all the gears. I was wondering if it is possible the transmission was geared different than the previous one. The van I have was a custom order van in 1988 and was purchased with many options. Such as engine and tranny oil cooler to pull trailers with so I thought maybe the rear end might be different.

ALL C-6s are 1:1 in third, all. There are NO variations.

You are not getting into 3rd. ANY Ford engine even with the steepest gears they made (4.11) will run 70-80 MPH in "D" if it's really in 3rd. Otherwise they are screaming about 55 in 2nd.

I'm with ModMech on this one. While you're at it, I hope that you're not overspeeding your engine in second gear? That's how you bend connecting rods!