You should be ok to have them on a slight lean if anything lean them so that the air is travelling 'downhill' into the engine.

Too much angle will f--k with the operation of the floats in the bowls though...
youll end up having to stuff around with the floats to make sure theyre holding the right amount of fuel.

uhmm yes i thought of that! The floats are an issue! I`m now thinking also if it clears the master cylinder!

I would suggest to go to bike shops or bike scrapyards that carbs are still on the engine. You can then get a general idea at what angle to put them on your engine.

How are you going to make the custom intake manifold?
Use TnPShows templates?

I already have the intake flanges in 10mm alluminium. I just need the tubes! What do you sugest?

This is where the tricky part comes in. You will have to check out the carbs to see at what angle they must be put on the intake runners to let the floats,etc on the carbs work without problems.
Bike engines tend to be at a slight angle to the front part of the frame. So I would think the bike carbs will also have to be at an angle towards the engine. That is why I would suggest check out bikes at a scrapyard to see what the angle must be + - not to make a mess of it.

Please mail me your setup's pics. I am really tempted to do the same but with fuel injection.

i`ve not starter yet, but i will soon and i will send pics!