Sorry guys, I've been off the forums for a while and didn't realise 2 weeks passed so quickly. Chances are its not selecting reverse now because of the reverse idler arm and shoe that I showed pics of before.
There is one other way to check to see if you can get reverse to work right without pulling the gearbox out yet.
There is a bolt on the side of the gearbox that has to align with the reverse idler arm inside. A lot of guys will unscrew this bolt out and not realise that when you look inside the hole, you need to get the thread hole for the reverse idler gear arm to align perfectly with the hole before you screw the bolt back in. If someone has undone that bolt and the arm has moved slightly and hole is off centre, you cant get the bolt back in properly and can actually thread it or jar the arm from moving properly. You need to chuck a screwdriver in the hole and align the holes up properly so you can screw the bolt in all the way to hold the arm in the correct position so reverse can be selected, otherwise it's just going to feel like you can never get into reverse... e.g. feels like something is blocking access to select reverse because the arm inside is not on the correct angle.
See pics. I would unscrew that bolt I have circled on the outside of the gearbox casing now, shine a torch inside, use a screw driver to get the hole to align properly, then screw the bolt all the way back in so it actually screws into the reverse idler arm nice and snug - not over tight. Measure how deep the bolt goes into the casing to be sure that it is actually going into the thread of the arm!!!
If that isn't the cause and you still cant select reverse gear then yeah, the reverse idler arm setup, shift fork rail end. lever that drops the reverse idler gear down or the shoe inside the reverse idler gear arm is the problem and you need to open the box up to the bellhousing anyway .