The external dimensions of the 400 are the same as the 350 (it was only 326 CI in 1967). The Pontiac V-8s had fairly stout crankshafts with large journals, and as such were reliable. Architecture is similar to a Chevrolet V-8 from the oil pump and lubrication system to the timing drive. The intakes were all of the air gap variety and thus used a stamped steel valley pan to cover the lifter/pushrod area. Intakes are difficult to acquire compared to other engines. If the stock intake is designed for a four barrel carb it would be more desirable. It would also be best to get a complete engine with all accessories since finding accessory drives and brackets would be difficult, and even more difficult in the UK.
Engine mounts are not difficult to adapt.
https://www.yearone.com/Product/1967-81-firebird/tb389