Thanks, pass this along to my friend who is the one working on the 68Before purchasing any mounts, look at the side of the block to see how many Motor mount holes are there, you said it is a later 400 block, but not what year. If it is a 76-79 400 Block they usually only have 3 holes in them for what was considered Firebird Mounts. 65-69 Pontiac V8's had either 2 holes or 3 holes depending on the original application they were used in, 2 holes for Big Cars & Lemans, Tempest, & GTO's, 3 Holes for Blocks originally installed in Firebirds.
If the engine is a 1970-75 all 5 holes will be tapped and you can put them in anything without adaptor plates, if it is earlier it will not be an issue for the 68 Catalina, but if it is a late block it may need adaptor plates.
Here is a link to the adaptor plates you may need