1984 Chevy S-10 Gauge Repair

I recently bought this truck. What I found out after I bought it was that it had a replacement 'junk yard' motor put into it. Whoever installed it, messed up the in dash tachomoter. According to it the engine idles ar 6000 rpms and as soon as I touch the gas peddle it pegs to the max, so it is really useless. What could you tell me as far as trying to repair this. I have had to work alot of other 'bugs' out of this truck, which kind of makes me feel taken already, but I do like the S10 trucks alot and would like to fix this one. I would really give you a donation if I could, but I am a disabled person (Ive had 3 heart attacks in the last 4 years) and live on disability. Thank you for any help you can provide.