I have an '81 Z28 and i'm trying to drop a new motor in. I recently bought new motor mounts so i tried to take the old ones off. The mount to block was easy enough but when it came to the mount to frame, it seems impossible. There is a nut holding each bolt to the frame but in order to reach the nut i would have to get a wrench into the crossmember somehow. I don't understand how this is possible. Please help.

 

Once you've removed the motor mounts from the block there's no reason to remove more than the single through bolt that attaches the motor mount to the frame mount. You'll of course need to take the weight off the mounts to get this bolt out by jacking up the engine, or using a hoist to raise it slightly.
The frame mount doesn't come off without chiseling the rivets, but not sure why you'd want them off.

 

How do i change the urethane pad then?

 

You have the rectangular clamshell type of motor mount that's sandwiched and bolted to the frame on all four corners, right? Are the nuts inside the frame spinning? They might want to have been tack welded to the frame, maybe not. Have you looked on the underside of the frame for access holes? One side, possibly passenger side may have holes. Have you tried to see if you can access the nuts from above, through the upper control arm area?

 

When I replaced the Engine in my camaro I had to fabricate a Wrench with a 90 degree angle to get in there to the bolt... it was tricky but it can be done

 

I've got an old set of offset box end wrenches I inherited from my Dad, and they fit in there fine. When reassembling I put the nut into the box end and put a piece of tape across the wrench to hold it while I start the bolts.

 

Had problems with replacing my motor mounts.. the removal and the insertion of the new mounts was difficult for me..

 

Even changing out just the engine mounts can be a exercise in frustration. I recently changed mine on my 427, and having done it before, I figured it wouldn't be a big deal.
I ordered new Moroso steel mounts from Summit, and when they arrived I jacked the car up and began removing the single bolt off each mount. Then I jacked the engine up and took them off the block. New mounts were wrong, so I decided to run to the local performance shop and buy a set of Energy Systems mounts. Took them home and compared them, and they looked OK. Got them on the block, set it back down and they hung up on the frame mounts! Jacked them back up, removed them and took them back.
Next I decided to just go back with stock BBC mounts. Parts store set them on the counter, and I compared them to mine; wrong type. After 15 minutes, and going through everything they had for a Chevy V8, I finally found two that looked identical to mine.
Went home and bolted them on the block. Set the engine down again, and the darn things were extremely close, but not enough to get thew throughbolt in! Jacked the engine up again, pulled the mounts and took my unibit and drill to the hole. After opening it up about 1/8" larger, I bolted them on the block, and lowered it down again.
After 7 hrs. of working, parts running, swapping, and lots of cussing, I finally had the engine bolted down. Had to go back with my torque chain from the frame to the driver's side head, but they're there, and I'm not going through this again for a long time!
Just a simple motor mount replacement.