When I shift my bro's 91 Eagle Talon, the gears grind if I push too far, clutch all

the way in. What's wrong? His car was the first other car I've driven stick shift, didn't really have problems adjusting from my 2008 Civic Si. The only thing I had a problem with is when I shift, the gears sounded like they were grinding when I had the clutch all the way in. After playing with the gear shift for a while, it would just randomly grind, can't figure out exactly why. I came to a slight conclusion that it was because I would push the gear shift too far up or down. But that doesn't make any sense to me. My bro just got the car, the guy who sold it to him rebuilt the engine, and put on a load of upgrades, new racing clutch, racing transmission, exhaust, intake, tuned the turbo properly blah blah. My dad said that the car doesn't have syncros or something, but wouldn't that mean I'd have to double-clutch when I shift or something? Please help me out.