These trans' are pretty simple, this is how we all learn, so, take one step at a time, take pictures, clean each part as it comes apart, set it aside when cleaned in the order it came apart, thenreassemble if any parts are worn beyond use, search here and other places.

I am sure you will find out soon why the reverse shift rail does not move. Some trans' need to be in a certain gear before the top cover can be removed. At some point you might find it better to replace it with a trans for which parts are readily available-420, 435, 465, 540 series, etc., might involve boring bellhousing retainer hole to a larger size, finding a new bolt hole for one of the mounting bolts.

Ed

'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires.
'47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle.
'54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed.
'55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.