I've done a 3.0 to 3.0 swap on my truck. It took me around three days to get the old block out, prep the new block then sit it back in the engine bay on mounts. It then took another few on and off days to get the exhaust and transmission reconnected, set the timing, get the hood back on, etc. If you are comfortable spinning a wrench and are willing to do research it's not bad. You will need a way to to move the blocks(I used my kubota tractor and a logging chain), an engine stand and some kind of workbench. I had to do a little extra work as the longblock I bought was a used one that probably came from a Taurus. This meant I had to swap heads and some other things to have the block setup for use in a B-Series.
As for going from a 3.0 to a 302... I'm in the process of preparing for this myself. Depending on the year/model of the donor and the year of the recepient will dictate how much effort is involved and how much custom work you will have to do to make it work. There are parts/kits out there for certain combinations and plenty of information but you'll have to chase it down. It helps greatly if you have a full donor vehicle versus having to grab/buy parts.