Thanks GTX - am starting to look at configurations
Kerick - did some digging.
Standard RA-5C carried 1430 US Gallons in the wings plus 1285 in the fuselage. The internal configuration in the attack version with Nuclear weapons was 1 x Special weapon and some additional fuel:
North_American_A-5A_internal_bomb_bay by
David Freeman, on Flickr
If the weapon is discounted an additional 885 Gallons in three tanks could be carried, giving a total of 3,600 US Gallons. In this configuration, subsonic high altitude combat radius was 1,310NM.
In the configuration of my model with the 4 x 400 Gallon tanks, the fuel load becomes an extra 1,600 Gallons - around .44 of the internal.
So, adding .44 of 1,310NM would give a radius of around 1800NM.
No need for conformal tanks........