That is a special rig. Please, buy it before I ask where it is.

Please more pics. That looks exceptional.
That's fantastic power for a nimble yet very rugged little Seafari. As Bushwacker mentioned, the Chevy small block was an option in the late '70s. The extra weight fore might actually be beneficial in holding her down in while planing 3-4' seas. Handling, and sea-keeping wise, I don't see a problem. I think balance and stability will be very good. Trim tabs are a must for your conditions. Can't wait to hear how she performs. Good luck with the purchase. That is a sweet score.