I performed this repair in 2013 on my heater core with the same issues you described.
The heater core sits inside a massive blower box assembly and is held in the box with a metal plate and 1 screw. Mine developed a pinhole where the plate had rubbed through the heater core. Some creative soldering to fix the leak and a rubber pad under the plate to prevent future problems.
The job took around 7 hours of actual work as the dash must be removed almost entirely. The lower parts, glovebox, radio, air bag, instrument cluster and a fair amount of the heater tubes all have to come out.
The dash itself can be loosened and lifted up....it does not need to be removed but many have done this to get more maneuverability. Not a fun job as there are plenty of areas of cut yourself and some spots are very tight for space.
I posted a few pics of my efforts on this forum.
http://www.delica.ca/forum/viewtopic.ph ... eater+core