Single disc conveters run about $500. Installation is simple if you have a transmission jack. Otherwise, it may cost you $100-200. The higher end multi disc converters will cost you $1500.
Stall converter specs are probably the second most debated mod, next to 3.73 vs 4.10 gears.
go with atleast a 3200 and if you plan on locking the converter durring WOT operation ( less drivetrain loss this way) then go with a multidisc.
installation will depend on where you live. where I work labor is 93 a hour, and I know I would not drop a tranny for 1 hour pay. I would expect to pay atleast 300 for labor depending where you go. My shop would be more in the 600-700 range to install it.