"Camshaft and lifter life can be increased dramatically by modifying the block. Drill and chamfer a 1/4inch hole at the base of each lifter bore boss except for the two at the rear of the block. Then grind grooves along the boss and connecting with the hole."
Is this a standard practice with rebuilders - will it prove a benefit - do you guys normally do it? what issues may I have +

Setting up the new CAM - what actually defines the amount of backlash or end play - should the rear welsh plug be fitted first - how far in the block should it sit?

Lots of question I know but I want to get it right 

It has been found that the additional holes in the valley are beneficial to providing additional crankcase pressure relief but provides no benefit to camshaft lobe or tappet oiling.  Here are links to previous threads on the subject.

Drilling additional holes for oiling and/or venting

Valley holes for aiding crankcase evacuation

The rear cam plug should be installed flush with the back of the block with the cup facing rear ward.  This requires using the correct plug and not a deeper dish style of plug.  If all is well, you should have the cam falling at least 0.020” behind the front face of the block before installing the cam gear hardware.

Here’s the link to the rear cam plug article that was in the Y-Block Magazine.

http://www.eatonbalancing.com/blog/2011/10/21/rear-camshaft-plug-installation/