More than likely adjustment is off. You kinda have to clock shifter to transmission. I kinda have the same problem I put my trans in and its ever so slightly off to where I can squeeze into first but I have to pull collars up and push to left a little bit while going into second.

Jack the car up, above dogbone/torture mount there is the linkage adjustment that's uses a 10mm or 12 (extension would help),

(This is how I always do it)

Get a helper to put shifter inside to where reverse is.

Go under car and loosen the linkage.

Using you hand mimick the motion of putting car in reverse by using transmission side of linkage
[twisting all the way towards transmission and pulling back toward cab of vehicle]
Making sure person inside doesn't move the shifter.

Once transmission has shifted into reverse. Tighten the linkage and your gear set should be back in order. Don't not over tight adjustment bolt.

Any question of quick help I'm on Facebook at Melvin Melveo Lunsford