Yes, the bomb carriers wwere normally bare metal, but were sometimes painted black, which very soon wore off, giving a tatty appearance of spots of black.
The positioning of the bombs sounds about right - the 1,000 pounders near the centre.
Looking at the wing bomb cells in you pic, they actually seem a bit short in length, probbaly the fault of the kit's engraving.
Dwight's suggestion is what I was going to suggest. If you haven't got 250 pounders, then shorten the extreme tail of the bomb body, and the length of the fins. I'm not sure of the details without searching, but there were shorter tail units available for British bombs, which might have been for this or similar reasons, and, of course, the shorter-finned US bombs, with 'square' fins and no tail 'ring' were also used later in the War, from about early 1944.