Mitsubishi Repair: 1990 mitsubishi clutch manul 5 speed, bolts and nuts, rusty bolts

Question
how difficult is it to replace a clutch

Answer
Carl,
If you do not have any experience with replacing a clutch, it will seem like a very difficult task.  You will need to be able to pull the axles, detach engine mounts and drop the transmission in order to get to clutch assembly.  Also, depending on your style of Eclipse(2wd or 4wd), you may need to remove certain framing members in order to access the transmission.  Having rusty bolts and nuts will increase the difficulty, and you'll want some heavy duty tools as well (breaker bars, pneumatic impact wrenches, transmission jack, etc.).  And especially on older cars, once you remove a older component, you might as well replace it with a new one.  Having new hardware available and investing in new parts like a slave clutch cylinder and shift forks can be a good idea.

A couple critical components that you should have before even consider attempting this job are the factory service manual and a backup car.  If this Eclipse is your daily driver and you cannot be without it for at least a couple days, I'd recommend you not attempt this job yourself.  But if you're looking to try something new and are able to do the replacement at your own pace and have all the documentation regarding this replacement, it's a terrific learning experience.
Good luck!