dont see that it matters just so the fit properly in the holder here is from the gm service manual
Valve Lifter Installation
Important: When using the valve lifters again, install the lifters to their original locations.
Align the flat area on the top of the lifter with the flat area in the lifter guide bore. Push the lifter completely into the guide bore.
Notice: Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.
Tighten Tighten the valve lifter guide bolt to 12 N·m (106 lb in).
It wouldn't matter if hole was in front or back your saying? Is this hole where the oil goes into lifter or oil drains out of?
on this picture where is the hole you are referring to located
I can't get pic to load here. Do you have an e-mail I could send it to?