IH 3500a wont move

I have a new to me early 80s IH 3500a that is giving me problems. The machine will not go up or down even small grades and when it does move it will only move in low gear with lots of throttle. The transmission is a syncromesh with a high/low selector and gear selector next to the seat. The forward/reverse is with a lever on the steering column. I am thinking it is most likely a bad clutch but wanted some advice or troubleshooting tips before I start pulling things apart. The problem seems to be worse going in reverse than forward. All the hydraulics on the machine are working fine, the machine just doesn't have any power to move. Is there anything else I should check or does this sound like a typical worn clutch?

Thanks,

Keith

IH 3500a wont move