You need a new battery tray and hold-down (any good one from Kragen etc. will do), an 18-inch negative cable, and about ten feet of 2-3 gauge or better RED battery or welding cable. If you can get one with a battery terminal on one end, so much the better; otherwise you'll have to solder a terminal on each end.
Put the battery just aft of the passenger side wheel housing. Bolt the negative cable to the frame right there. Run the positive cable over the wheel housing, down under the quarter window, into the passenger side door sill, into the kick panel, and up and around to a point on the firewall where you can drill a 1-inch hole, Put a 90-degree plastic conduit fitting in there (from Home Depot, etc.) and run the cable through it. Up the firewall, around the shock tower, to the solenoid. Cut to length there and solder on a big ring terminal.
Tie the cable down with loops and screws at 12-18 inch intervals,and pad it heavily (I used bike inner tube and wire ties) wherever it even gets near an edge.
Then, if your car is like most, hack out the rusted remains of your battery tray and front inner apron and replace the latter.
You might want to put in a racing-style cutoff, using a big plastic key-style switch, in the trunk while you're at it. Makes cutting the power easy and is a simple anti-theft addition.
Just what dotbomber said. I bought the taylor kit from summit and that gave me 1 gauge cable with one battery terminal already attached, the other you attach yourself once you have it cut for your car. With the kit I also got the cool brushed aluminum battery box that spices up my trunk. Unless you want a battery box like that you should just round up the parts on your own. Oh, I also ran some 1/4" tubing to vent the box so the fumes don't build up. The engine compartment less the battery looks alot cleaner, but its hard to put your finger on what it is.