the 'vortecs' that are prone to piston slap are the gen III vortecs (5.3's and 6.0's, etc), not really the gen Ie SBC vortecs....more than likely the ticking you're hearing is a lifter or something that has bled down and takes a second to pump back up with oil...like was mentioned, if it goes away within a couple seconds, you're probably okay, if not you'll probably want to look at a rebuild sooner rather than later...although if you've got a decent oil leak, which it kindof sounds like, make sure you are frequently checking the oil level, if it gets too low then you can oil starve the valvetrain and it'll make a similar knocking/tapping noise which will take alot longer to go away, and if the oil is extremely low, it won't go away...
to answer the questions in your original post, the only headers that will work as a bolt on without modifying the exhaust are shorites, but they won't provide much gain, so for their cost, its not really worth it IMO. Long tubes are good, but they will require a good amount of fabrication to work. Thorttle body spacers are completely worthless for performance on any 96+ truck since our engines are port injected. they do absolutely nothing unless you're trying to use them for fabrication purposes (i.e. tapping them for nitrous or using them for clearance or something). The stock air ducting from the filter box to the throttle body is perfectly fine for your setup, it can supply a good amount of air without a problem, much more than the stock intake manifold can use effectively. personally I'd ditch the K&N, there is no conclusive evidence that they give any power gains, and from my experience, they are piss poor at filtering, so they let a LOT more contaminants into the engine. If you want to read a little bit on the subject, here's a test I did of various air filters, the K&N was the second worse filter in regards to letting contaminants through :
http://www.gmtruckcentral.com/articles/air-filter-study.html
personally if you know you're going to be pulling the motor somewhat soon, I'd say save your money to use good parts for the rebuild/swap and build the engine the way you want it...but if you really want a little performance gain until you get a rebuild or engine swap or something, then really the only thing I'd suggest is to get a PCM tune.