Your stock mounts should be fine.

However, you should have gotten an ODB1 B18C1. There are two types of ODB2 in the US: 96-98/OBD2 and 99-01/OBD2. They are electrically the same wrt sensors, distro, injectors, starter, and alternator; the only difference is the engine harness ECU plugs and the actual ECU. So, with your OBD2 to OBD1 swap, the issues are:
1) Injectors - not plug compatible
2) Distro - ditto
3) 2nd 02 sensor - not present
4) there's something else can't remember right now

There are a number of OBD1 ECU's that can run the C1, and you don't have to use the P72/GSR ECU if you replace the intake manifold with a single-butterfly one (the route I would take). Also, going the ODB2 to ODB1 jumper route is another possiblity.

Regarding the ITR catalytic converter...this will bolt to the GSR exahaust header collector pipe, but is not designed for the GSR and may be too restrictive. I would go for a GSR specific high flow cat.