If your rear end is stock then it is probably a 4:56 ratio. At 75 MPH the engine would have to be running pretty flat out and the one your looking at is not a high RPM engine. Someone will come along shortly with the gear selector website info so you can figure it out. Also, with 5K behind you I don't want to be in front of you when you are trying to stop. The 56 Y/O brakes just are not going to do it well at all.
You did not say what tranny you have so I cannot recommend one as you may already have it but it would likely need to be an OD unit.
I think you may need to rethink this whole thing with the weight and speed your wanting.
