Was that header listed as a 4-2-1 or just a header? I'd contact seller.
The primary runners don't have to be very long because the exhaust pulses are split between the two header parts, just as long as they arrive at the final collector individually.

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'S all for now
Vic

I would think 4 carbs or two 2bbl carbs side draft would be best. A manifold straight in no bends and ram pipes will give a good ram effect. Then if you get the air/fuel mix right your golden. Two 1bbl carbs into 4 cyl like the su's is a proven system and seems to be easy to set up. Getting the mix in and getting the spent gasses out has more to do with the head and exhaust. Bottom line if you got the right mix going in at the time that cyl wants it the cyl does not really care how it got there. So if you are a Weber dcoe fan or a motorcycle carb fan it might come down to that or might come down to what you think looks better under the bonnet. I agree with Carter unless you are going to race it this might be splitting hairs. If you are thinking of going this far I would just go FI.