Going through the access hole in the floor would be the easy way to go. get yourself a small funnel to fill it with, and only use that funnel for brake fluid. I would suggest trying to get as much of the old fluid out of the master cylinder before you add new, then open the bleeders and let it gravity bleed (adding fluid to keep it from going dry) before you try pumping it throuhg. That will help keep the new fluid from getting contaminated.
On the cables, I would suggest using PB Blaster or Kroil over WD40, far supior penetrant action. Also, if you still have the original cables, they may have grease fittings on them. Once you get them working free, try shooting some grease throuhg those fittings, if present. You should also pull the drums and check the sliding points in the brake system, lube if needed. On the rear, lube the parking brake actuator as well. I suggest using Permatex synthetic caliper lube, the stuff in the green tube/bottle. Good stuff. I get it from the local 'Zone.
