My engine is L28 with N47 head and headers,

What to buy 40 or 45 dcoe?

Its just for street driving.

45's would be better for 2.8litre because you can expand air flow easier if you bore the block in the future.

Chokes (venturi) from 32 - 34mm are recommended for l28 and 45dcoe. 32 gives more lower end torque.

34mm is recommended limit for 40dcoe.

see chart below 33mm choke is crossover point where 45 starts to outperform 40 with same choke size

Be sure to read every post you can find to get your set up correct for your driving needs.