The cluster doesn't pull straight out. Look at the blue/green-ish housing plastic, parts of it sit behind the dash sheet metal. You have to push it in a bit then lift and pull to get it over the dash lip.
Best way to do the cable IMO is to get the cluster housing free from the sheetmetal, then leave it hanging. Go under the hood, and push the speedometer cable through the firewall a few inches. That will give you enough play to reach behind the cluster and DEPRESS that tab illustrated above. Push the tab in towards the cluster, and the cable will pull straight off.
Bulbs are a mixture of 168 and 194's. I don't know if the '88's had the one piece bulb/holders, but the bulb type is printed on the holders in that case. The hotter bulbs tend to warp/melt the plastic. Yes, people have used the hotter bulbs with no ill effect, but I've seen factory clusters with loose bulb sockets (and discolored plastic) from the heat of the stock bulbs. I went LED for multiple reasons, that was one of them.