The comp XE268H cam is likely to much cam for a low compression 360 (a stock bottom end LA360 is going to be at or under 8:1) and you will be giving up bottom end. The XE256H or XE262H would be a better choice.
Any dual plane manifold will work even a stock iron one with the build you are planing.
If you are going to a square bore carb like a Holley something in the 600 to 650 range would be a much better choice for you goals. With the 750 you will give up a lot of low end throttle repsonse and driveability with little or no gain on the top end.
Definately stick with the 1 5/8" headers for the low end torque. Tube diameter determines the rpm at which the torque peak occurs with bigger tubes having the peak at a higher rpm.