Will 87 Si gauge cluster fit 85 DX?

yes it will fit
and because you are replacing the wiring harness for the fuel injection, you'll have all the correct wires

the mounts are all the same I am pretty sure.

I should mention one thing I've been learning the hard way. It seems many of the connectors on an '85 are not compatible with the '86. So far, 3 of the 5 connectors I've dealt with-- on the throttle body, radiator, and steering column combo switch-- were different. You can just pull the pins out of the connectors and swap them, if you have a matching male-female set. -scott