What size Holley 4150 or similar for engine?

Nothing wrong with the carb you have.It may need to be tuned for best air/fuel ratio.If you just have to be fancy a 750 double pumper would get my nod with the 4 speed and 4.10 gears.The street demon 750 would be fine if you want/need a choke.

I bet that cam would run better advanced to 106 or 104 intake centerline.