I'm not sure I agree with Dominik about the engine mount length - an O-320 and O-360 is the same length and weighs with 10lbs of each other, so there is no need for a different mount.
You can also choose how to mount the injector - if vertical go with an O-360 cowl as the injector is nearly the same length as a 360 carb (although you may need a spacer to get the FAB in the right place vertically, if you use one). You could turn the injector horizontally, I think AFP supply an elbow, and you could use a Rod Bower filter, that would allow you to use a flat bottom ('injected') cowl. There is also the Sky Dynamics 'Injector Relocator Kit' - often used on Pitts. Which ever way you go some glass work on the cowl is no big deal. I don't think there are any other significant differences?
From the FF kit perspective, its not nearly so critical as many people don't buy the full kit. Van's will send you a contents list, and you can select what you want. Call Larry Vetterman and talk about exhaust pipes. A baffle set for a 320 will fit. Your control cables lengths will be bespoke so no point in buying those in a kit. I don't like the Van's flexible hoses and I think there are better engine mount rubbers available, so I have bought them separately.
If you know the history on the engine, and can get it for a good price, I would go ahead and not worry that it is a slightly 'non-standard' set up. Builders have been installing engines that are not those 'recommended' by Van's since the early days - there's plenty of experience around to help you over come any problems that you might encounter.
Pete