How to replace motor mounts.

8 hours is a fair estimate IMO. It probably wont take 8 hours, but it could if all 4 bolts are seized. It generally takes me 3-4 hours on most trucks. I've done in about 2 hours of the bolts zip right out (rare). But, if I tell a customer 4 hours and it takes 8, they hate me. I tell them 8 and end up charging them for 4, then I'm a hero.

Generally, no, you do not need to take the bolts out of the frame or flywheel housing unless those brackets need to be replaced. Be warned on the flywheel housing, they use strong loctite on those. You will need to heat them up or risk tearing the threads out of the aluminum housing.

Typically, you take out the bolts going though the rubber mounts, jack the back of the engine up and replace them. Sounds easy enough, but the bolts are most likely seized and the the spacers are probably seized to the bolts. Lack of room, wiring harnesses and air hoses make it very difficult to torch them out. You may have to get a lot of stuff out of the way and put a lot of wet rags all around to protect things if you need to torch them out. In some cases, there is enough room to use a cut-off wheel.

Before I get started, I spray down the bolts with penetrating oil, then use a long air chisel flat bit on the bolt heads to vibrate the crap out of them. This helps get the oil worked in and loosen things up (sometimes). I use an impact to get the nuts off the bolts. Then I use the air chisel from the bottom to try and push the bolts up through (this works about 50% of the time). If they won't come out becasue the spacers are seized, it's cutting time.