Confirm hardware specs via manual, whether the mk1 accepts PC133 or only PC100 RAM. Then look at the fine print on your RAM modules. At first glance both look the same and fit the slot, but if the mk1 only accepts PC100 then PC133 won't be of benefit. You also can't mix and match these. Also some motherboards only accept single sided RAM modules, not double bank, may be why it's only seeing half RAM.
In regards to exceeding maximum reported RAM on the mk3, old manuals only indicate how much RAM worked based on current manufacturing at time of print. For example, if 256 RAM modules were the largest available at print that doesn't necessarily mean a 512 MB module would not work.
The specs on your modded mk3 are nice, you will have no problems running *Core.