First, what 6 guages are you looking to install? I am wondering what 6 items you could possibly monitor on that year of car. I figure, Oil pressure, Water temp, Amps/volts...and maybe tach?? There is not much else left to monitor unless you are going for oil temp (?)
Is it a hassle? That depends on your mechanical aptitude and how neat of a job you want to do. Most guage kits come with fairly detailed directions, usually the biggest hassle is getting through the firewall. You will have to plumb into an oil pressure galley for the oil guage, a water port for water temp and tap into your alternator/generator for amps/volts. Then hook into the distributer for tach. Mount all of the guages someplace on the dash where you can see them, but still out of the way, or in the dash if you want to hack up your dash. Run all of the hose, sending unit and electrical leads, and hook into your light switch if you have illuminated guages.
Expensive?...it could be. On the low end guages are $20 (+/-) each, a cluster could run hundreds, the better the quality, the more they cost. You could probably pick up a simple 3 guage cluster (water temp/oilpress/volt) from your local NAPA for around $40-$60.