OK, I will be prep'ing one. I drilled a 3/32" hole in the right lifter gallery core plug right behind the cam retainer plate and will drill a bigger one in the tensioner so oil will feed the chain better. Ive done this on a few smallblock engines.

These Magnum blocks main bearing feeds are already drilled out bigger than the Herb McClandless spec's of 9/32".

they are 5/16" from the right lifter gallery feed down to the main bearings. And all but one of the oil holes from the oil pump mount up to the 90 degree over to the oil filter plate are drilled at 1/2 inch, BUT the return from the filter was only 7/16" and VERRRRY rough, looks like what is left over when a drill breaks off while boring. NASTY.
I drilled it out to 1/2" and it is now a clean bore.
I am using the Magnum 6 hole oil filter plate but drilled the 6 holes from 1/4" out to 13/32" , Just because I can.

My 5.9 Magnumn block has a Production date stamp of 24708 and "Hecho en Mexico" . I thought that the 5.9 stopped in 2003.