Brian,
There are several reasons you may not be able to engage reverse.
Did you check the operation of the reverse lockout mechanism?
Yes, the 5th/Reverse detent spring is stiffer and therefore causes more resistance. But also, the centering spring puts a lot of preload on the 5th/reverse shift rail and that makes it more difficult to move.
Steve's note that the gears themselves have to engage is correct. The reverse idler has to be moved to engage between the reverse gear on the input shaft and the reverse gear on the outside of the 1-2 slider. If the reverse idler teeth clash with either of those the gear resists engagement. Therefore it takes a fair amount of force to engage reverse when moving the rod by hand.
Spinning the lay shaft offers no assistance in aligning the gears. The shaft turns inside of the gearset unless one gear is engaged. So you must turn the input shaft and that can be reached from the bellhousing side. If reverse is difficult to engage try turning the input shaft to re-index the gears. It may take several tries.
You should always inspect the components carefully. The internal translator linkage especially. The lug at the end of the translator fork can get bent and cause all kinds of shifting problems. Also, the main shift rod and the shift rails can get fouled. Inspect them for damage as well as the case bores. And make sure you preoiled all of those parts on assembly. If the parts don't have lubrication they will not want to move.
Also, make sure the 5th gear slider isn't hanging up on its hub.
Then there is there is the issue of the unit having been rebuild previously. You never know what a previous rebuilder might have done. So inspect everything and make sure they haven't done something like use the wrong reverse fork.