I've got an 87 firebird with the 305. This guy gave me the gauge cluster from his '87 but (forgive my ignorance) I need to know what I'll have to do to make the swap. His was a V6 and the speedo goes to 120. Mine only goes to 85mph.

Is is do-able and how much trouble is it gonna be?

 

the speedo might be a little off but aside from that, im guessing his is the rally cluster and yours is the base cluster with dummy lights and such... Well if u indeed have the base cluster and want to use his then your going to need sending units for the temperature gage oil pressure gage and im not sure about the volt meter, that might already work when you plug the thing in. The tach is probally pre wired but since it came from a 6 it prob redlines at 6k, where as the 305 redlines at 5, soo theres an issue. oil pressure might be diff on that cluster too, as 305s read 0 30 60, from left to right, and the v6 might read 0 40 80 from left to right, so the issue with that is if u bought the corect sending unit for your car and his gage does read 0 40 80 then the oil pressure may seem to read on the low side. with all of that mumbo jumbo asside there is a trim piece which you might need (because i assume your converting from base to rally cluster) which has 4 holes in it to make the cluster appear as if it has seperete gage pods...

----ok the actually swap part isnt brain surgery, there are a bunch of 7 mm bolts holding that trim peice on and that peice is the only thing in the way of the cluster. once you take that off there should be some more 7mm bolts holding the cluster to the dash, take those off and pull at your old cluster until it comes off, and push his cluster on and re assemble. the sending units are not necessary but without them your oil pressure and temp gages will just be grounded out (the gage will move all the way to right and read off the scale) hope this helps you
-here is an old pic of my dash, which has the trim piece you might need and a view of the new gages,