I started out with stock I6 perches for my 66 Mustang and stock I6 mounts on the 250 six I'm trying to fit.  I had a lot of trouble getting even those to work until I realized that I had the right mount on the left side and vice versa. Once I got that figured out, I let the engine down so it sat on the perches.  The valve cover was too high - my Monte Carlo bar hit it and the ends of the bar were about 1/4 inch above the inner fender where it's supposed to bolt up.


 Next I tried just spacing the mounts down so that the bottom hole in the mount was in the top block boss hole (had to try).  It definitely lowered the engine but the mounts never seated on the perches because the oil pan was sitting on the sway bar, the cross member and the steering center link.  The sway bar and cross member were not big deals because I know I could put spacers in to lower them but the center link was a killer. So I compromised and made a pair of brackets that would lower the mount on the block by about half the distance between the holes in the block - a little over an inch (the black part on the lower left). 

 I drilled 2 holes in each bracket, using the engine mount as a guide.  These would be used to attach the bracket to the engine.  Then I marked two more holes.  The lower one was half the distance between the two already drilled to mount the bracket.  The upper one was exactly the distance between the holes on the engine mount. I welded a bolt to the back of the second-from-the-bottom hole on the bracket.  This mates with the bottom hole on the engine mount.  I again used the engine mount to define the location of the last hole, which would bolt the top of the mount to the bracket.  Lastly I drilled a new hole in the engine mount, matching the location with the third hole from the bottom of the bracket, so that I could through-bolt the mount and the bracket to the engine block.  During the test fits I discovered that the lowest bolt, the one holding the bracket to the lower block mounting hole, interfered with the mount.  I found that a button-head fastener would not interfere with the mount pulling in tight against the bracket on the bottom.  I sourced a couple of those from my local hardware store.
 After fitting them in and doing a little adjustment, the mounts have lowered the engine by about an inch, giving me 3/4" of clearance between the bottom of the Monte Carlo bar and the valve cover and plenty of clearance between the oil pan and the center link.



They've held up well over 750 miles so far but I'm considering them temporary until I can fabricate something that's more solid.  I have a dead floor jack in the corner that has some thick, stout steel sideplates that I think will make good material to make a more solid mount.  More on that as I get to it.