It's not that difficult, just tedious. Take your time and note where everything goes as parts are removed. It helps to tag/label things, especially hardware and any electrical connectors you have to disconnect. Make sure to take a lot of photos along the way. Once you get the dash components out of the way, the heater assembly and core are easy to get to.
I've just about finished a complete swap of the dash from my donor '89 FJ62 into my '86 FJ60 project vehicle. I'm also swapping most everything else over from the FJ62's interior, including the rear heater, complete wiring harness, power mirrors/windows/door locks, etc., but not the headliner. Plus brand new carpet from Trail Tailor. While at it I'm installing an older Pioneer AM/FM/cassette player along with a new Pioneer amp, Pioneer speakers in the front doors, and MB Quart speakers in the rear cargo area. I'm also upgrading the power window and door locks circuits with relays. So far so good, just waiting for the weather to warm back up a little and the snow to stop flying.