Solid vs polyurethane motor mounts, and is a tubular K-member worth it?

Mounts: adapter plates like ICT's accept rubber, polyurethane or solid mounts (and later clamshell mounts). The trade-off is deflection vs vibration. Speedway's writer chose solid mounts for a track-oriented build because he didn't want any deflection and the extra vibration didn't matter. For a comfortable street car, rubber or poly isolates more.

Tubular K-member: we don't have a source that evaluates K-members, so we can't say whether one is worth it for your car.