Welcome to the forum. We're glad you've joined us here and that you're asking for advice before making a big mistake.
Do not use any rubber or resilient material between the engine and the bike frame. This is a subject that has been discussed here many times. You want the engine mounted solidly to the frame. Rubber, or any soft material will NOT eliminate or reduce vibrations but in fact will amplify them.
Yes, motorcycles and cars use rubber mounts but the type of mount is not the same as most builders try with a motorized bicycle engine. They incorporate a rubber sandwich that is a different concept that putting rubber in the mounts of your bike engine.
Rubber in the mounts will only transfer the vibrations to the mount fasteners, which will eventually fail because they are taking all the stress. Mount the engine as it was designed to be mounted and forget any rubber in your mounts.
As for "custom tanks" many of us use motorcycle fuel tanks that have been adapted, modified, to work on a bike frame. What specifically are you looking at as far as a "custom tank"?
Good luck and keep us informed on your progress.
Tom