Enclosed pictures should help. You should have two dimples on the slanted portion of your firewall (engine side). These are the marks where you will need to drill two 3/8" holes for the "T" bolts which hold the radio to the interior firewall.
You should have an antenna lead which comes down inside the "A" pillar. This connects to the antenna lead that comes out of the radio. If you don't have the "A" pillar antenna wire you will need to add an after-market antenna under the car or use the spare tire or add one to the outside on the cowl. However, this will require changing the radio antenna receptacle to an RCA outlet (easy to do).
If you look at the pictures carefully you will see the plug outlet which has a speaker plug in it. If you want to use this outlet for a speaker you will need to have an overhead speaker. Unfortunately, only a '35 will work and they are getting hard to find. Another option is place a speaker inside the radio under the cloth cover on the radio (you may already have one in there). Hook up the dash controls and the power feed and your done!
All-in-all, this is a real simple job.
Hope this helps.
Tom