I had the same problem, in that "Mustang mounts" didn't really solve my problem and I tried a few mounts before I discovered the real problem. It doesn't appear that you have a Dart block, but you might check this out below.
I have a Dart SHP small block Ford 302 engine bored and stroked to 363 cid. I am using engine mounts from a 1978 Ford LTD with a 302.
When I first installed the engine, the engine mounts were about 3/8” too wide on both sides. After much frustration and searching for other engine mounts, I finally discovered the problem when I pulled the engine out so I could see the mounts more clearly.
The Dart block has some ridges by the engine mount points that are evidently not present on the Ford blocks. These ridges have been milled back a little bit on the flat of the engine mount points, but they protrude out too far to clear the rest of the engine mount.
Because of this, the engine mounts contacted the rib by the engine mount and prevented the mount from fitting flat on the engine; thus, the engine mounts were too far out where the mounts connected to the chassis.
In order to get the engine mounts flat, I had to grind off part of the rib.
There were some edges on the engine mount that needed to be flattened as well.
It now fits perfectly.
I sent this to Dart, but they didn't respond.
Let us know if this is your problem, but try the 1978 Ford LTD with a 302 engine mounts.
-Bob Woods
Tornado GT40 in Texas