perry take a picture of the linkage and the stabilizer bracket. Its also possible the syncro for first gear is bad or there is shift fork issue. did you get this tranny from a known working car? Did you open the case to inspect prior to install?
i bought it from a forum member. i have to unbolt the gear linkage and manual put in in 1st gear and see if it gonna pop out. gonna let you know. i im going to post pictures