I have recently swapped out an idiot light cluster to a guage cluster with tach( both 1980). The tach works but reads 3000 rmp at 70mph ( with 2.73s). This seems incorrect and i am assuming my rpms are actually around 2250. My question is are there seperate tachs for 6 or 8 cyl? or is there an easy modification to convert 6 to 8? IF this were a 6 cyl tach would it work but simply read higher rpms? Will a tach from later years work if i can find one?
Thanks in advance!

My question is are there seperate tachs for 6 or 8 cyl?
Yes, there are two different tachometers. One for V6 and one for V8.

is there an easy modification to convert 6 to 8?
No can do. There isn't a switch or jumper.

IF this were a 6 cyl tach would it work but simply read higher rpms?
Yes, it'll read 25% higher than a V8 tachometer. At 70 MPH you should probably be reading around 2400 RPM based on a 26" tire height instead of 3000 RPM.

Will a tach from later years work if i can find one?
Any V8 tach from 78-85 will match. 86-87 will work too but it's a different font style and the colors are slightly different.

Here is the 78-85 tachometer with the clock.

http://elcaminocentral.com/users/elcamino/pics/78_85_tach_clock.jpg

Here is the 86-87 tachometer.

http://elcaminocentral.com/users/elcamino/pics/86_87_V8_tach.jpg

also, having a good ground will help with some tach prob's. Just installed one and worked wierd; about 30 minutes later and a good ground made the difference.

You could mount a tach in it's place I found a small sunpro that was about $40 and went to 8000. worx nice.