ITR pistons in an ls block will give you lower compression than it would with p30's. If you want big compression that CAN be done on pump gas, go with ctr's. The rod's have to be cut to fit the ctr's and you might as well spend an extra 60 bucks on ARP rod bolts just for "cheap insurance" purposes. As far as the block goes for machining, just keep it stock bore you will be fine. The stock bore is 81mm. Have a shop hot tank the block and hone it. They will look for other things (if needed), like a ridge ream, line bore... etc. But if the block is in good condition, a clean-up and fresh hone is all that will be needed. From there, order your pistons in 81mm form. NOT 81.5mm. Its also a good idea to have the rods shot-peened to make them stronger. As far as the rest, just replace the main and rod bearings, new rear main seal, new oil pump and water pump, both from a 96+ gsr so you can use the gsr timing belt.