IMO, lay the grill down at a steeper angle than you have in the photo. Are you planning a tube frame, I hope? Given the age of the original frame, it will not take kindly to be sectioned.
Go ahead, ask me how I, unfortunately, know....
Too bad you already have a modern V8 for it. What would be cool would be a flathead V8 with a B&M supercharger, Edelbrock aluminium heads and a pair of 4bbl Carbs. Authentic Rat Rod that is.
You know, way back in the day, the big Cadillac engine was the power plant of choice. Coupled to a three speed non-synchromesh transmission.
Sort of hard to drive, but that was the point of a Rat.