You can use the Terminator heads but like I told you on corral(assuming it was you), the timing chain guides(non-fixed) between the two motors are different. If you want to use the Terminator timing set, everything else will work but the curved guides attach to the block differently. Teksid uses a bolt/bearing arrangement, while the '03 uses a pivot pin arrangement. You can just buy the pins and thread them into the block and use all the '03 stuff.
Make sure to use the correct crank gear/trigger wheel arrangement also. Thick gear/thin wheel, or thin gear/thick wheel. Thick gear and thick wheel will not fit behind the timing cover.
'98 MarkVIII should have the hi-flo oil pump, so you're good to go with that. The early Marks had a lo-flo pump that wasn't up to snuff. You'll have to make sure to use the Cobra pickup tube, the GT tube is too small and you will have a huge oil leak from the tube to the oil pump, not good. Not sure if the Cobra pickup tube fits the GT pan, you might need a Cobra oil pan also.