Hi Gary,
The Blackgates Clarkson drawings and casting are the route to go by. The NRM have lots of drawings available, and there is a complete set of drawings for the LBSCR H2 Atlantic which devolved from the original Ivatt drawings, and which is being recreated in fullsize on the Bluebell Railway.
I have a GA from Doncaster of the Ivatt Atlantics as per Maisee, but LBSC took some liberties with his 3.5"g design.
I have not seen the Clarkson drawings but would imagine they are very accurate but not easy to interpret unless you have a lot of background knowledge.
W A (Bill) Carter built his 5"g Gold Medal winning example that won the top awards in 1967 and 1968 if you refer back to ME of this period. It was also a working miniature that proved itself on the track on the SMEE exhibition track for many years and at Beech Hurst.
I would also particularly recommend you acquire the RCTS LNER volume relevant to the large boilered GNR Atlantics and this will give you some appreciation as to the Richardson balanced slide valve version as to the later piston valve version.
Cheers,
Julian