My neighbor has a 69 with a muncie close ratio transmission. He had installed the trans after a rebuild and adjusted the linkage while the car was on jack stands.There was no problem engaging all gears. Once down from the stands the transmission won't go into 4th gear. In addition the transmission seams to whine after warming up. Anybody have any thoughts?

Driveshaft may be too long jamming the output shaft into the trans, an idea.

Place the car onto the drive-on ramps and look underneath as one is moving the stick. May have to readjust the shifter. There should be a 1/4 inch hole to insert a drill bit thru the shifter box and arms, then adjust rods to slide into place with trans in neutral.

Close-ratios whined, due to gear teeth angle is straighter than other models.

Also, if it's a Hurst aftermarket shifter, make sure that the shifters "stops" are backed out enough to engage all the gears.

There were two different close ratio models, M21 and M22. Only the M22 had the straighter cut gears. Very desirable, but the excessive noise is why they got nicknamed the "rock crusher".