There will definitely be more harmonic transfer changing to solid body mounts. Especially from rubber. New cars aren't a good comparison, they are very well engineered and offer great isolation from harmonics. If it's a cruiser or a car show queen, rubber is the way to go. If you plan on letting it all hang out on a regular basis, Things need firmed up. You can fight the harmonics by using rubber motor mounts, trans, steering coupler, etc.
Thanks for the replies, kinda echoes what I was thinking. This is a driver but all the suspension and drivetrain are rubber insulated so I'm hoping the transfer will be minimal.
Jeff-
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You remind me of the timing on a turbo engine...