Today I spent all day drilling and tapping the block for 1/2" studs. I am pleased to say that I have, as a trial, installed the studs and the heads and it all fits.
I resolved the issue over steam holes. Edelbrock replied tonight saying there are only two holes next to cylinders 1 and 5. However advice from another professional engine builder is that they are not required and Alan Mann actually blocked over various passages. The purpose of the holes is to allow trapped steam to shortcut the route around the block and back up the heads - but on the GTD40 the engine is tilted down at the front so if anywhere the trapped steam is at the back, not next to 1 and 5. Since I have installed a bleed behind cylinders 4 and 8 in the manifold they do seem redundant. As it happens there are 2 already on the block - so I'll leave these.