273 to 383 swap

66-72 B body, C body, and 70-74 E body mounts aren't considered biscuit mounts. With those, you just use stock mounts and brackets for a swap. See 1st pic.

A bodies 72 and earlier use a biscuit style mount, see pic 2. You can't use a stock big block mount bracket with that.

To fabricate your own mounts or motor plate, lower the engine/trans in the car and bolt the trans to the mount. Then, with the exhaust in place, move the engine left to right to get the best fit.