On your 1955-57 Chevy car, the front motor mounts are at the front of the block and the rear mount attaches to the bellhousing. This works fine for original use but makes it hard to swap in a different trannsmission or add belt driven accessories to the front of the engine.
Use with engine cushions 910-18012 and transmission crossmember 916-28910 listed below. No welding, but drilling required.
720-4175 Instructions (PDF)