Primary size: large enough ID to cover the port shape. The #1 & 6 cylinders can be round (1-3/4"?), the siamese #2-3 and 4-5 may need a bigger round primary (1-7/8"? 2"?) to be flattened a bit to match the port rectangle on its longest dimension (the diagonal).
A tuned length is only going to work better than a simple independent system if the overlap triangle area is big enough - but I have no idea what that is for this motor, perhaps 75°?
Primary length: the Bell formula is a useful start:
850 × (XO point + 180°) ÷ RPM, minus the exhaust port length from the valve seat to the flange in inches.
Example: exhaust opens 70° BBDC; add 180 = 250; times 850 = 212,500; divide by 5,000 RPM = 42.5"; minus 3" port = 39.5" flange to end.
Join 1 & 2-3, and 4-5 & 6 separately.
