Use carby cleaner to clean it, water to wash it out and compressed air to dry it.
If the old injectors are stuck in the rail be gentle removing them, as the injector seats on the rail are fragile and can be bent easily. Bending once or twice is ok but they'll crack off at the rail eventually.
You can check the injector baskets in the old injectors by removing them with a screw. They do block up and are easily replaced.
If you're running 98 and haven't recently switched to E85, I would leave the fuel filter as it'll more than likely be fine.
If you're gonna replace the fuel pump and intend to upgrade in the future, bite the bullet and get a Walbro 255 installed now. You've already bought another stock reg so that tells me you don't intend to run anything bigger.
There's my 2c so take it as you will.