30mins is quick to get the dash out, never mind the heater core, took me much longer than that, and i thought i did it quick
The dash wasn't to hard, but getting the heaterbox in/out was a pain in the ass.
Just get a new copper heater core from NATRAD... think its teh same as 300zx... just take your old one to your local natrad dealer and show them and they can sell you a new one, not expensive, can't remember the price, $100 or $150 maybe?