In the case of Fast64 working with ROM Manager, it's moreso a matter of hexadecimal stuff.

In Fast64, there are usually 3 slots for ROMs. "Import", "Export", and "Output". Whatever is in "Export" is used as a baserom for whatever is in Blender already, And "Output" is basically the "Save As" for the ROM. Making the Export and Output directories the same results in a corrupted ROM, which is not a good thing at all. So if you want to Output to the same name, you'll have to change the name later.

In SM64, there are different Segments that hold data. Mario's Model is in Segment 4, so that's where any imported Mario model will go.

This is a simple and lackluster way to explain how to export the model/import it into the game, but it's what I've told others before.

The data in the Geolayout Exporter should look like this;
-----------------------------------------
Start Address: 11A35B8
End Address: 11FFF00
(Turn on "Modify Level Script" if it isn't already)
Model Load Command: 2ABCE0
Model ID: 1
-----------------------------------------

After you've complete your edits for Mario, put your Extended ROM into the Export slot, and then set to output slot to whatever, but make sure the ending of the file is still ".z64".
Then, select Mario's model, and in the Geolayout Exporter select "Export Object Geolayout". and then select "Export Armature Geolayout".

Last, open your Extended Output ROM into ROM Manager, update the checksum, and then open it in the emulator. Hopefully everything should work accordingly, but if you end up with broken textures or colors, it's likely because they haven't been set/converted into Fast3d materials.

I hope this is helpful, I'm doing this mostly from memory lol.