I've got a stock 350 in my '85 K20 and I just bought a pair of Summit cylinder heads (# SUM-152123) w/ 2.02/1.60" valves but otherwise should be a direct bolt in replacement.  I wanted to replace the lifters, push rods and rockers (not the cam however).  I believe I have a flat tappet cam in the truck, and I want to go to 1.6:1 rockers for a little bit more lift and I was wondering if this will cause ANY problems (valve clearance for example) or if it will necessitate longer or shorter push rods.  I need solid advice so please don't give me advice if you're not absolutely sure of what you're talking about.  THANK YOU!!!  

Ok. First off, why not change the cam? Get the most out of your heads, and while you're at it, convert that flat tappet cam to a hydraulic roller? Yes, your motor is a flat tappet. GM didn't start putting roller cams in trucks until 1996. Cars were in 1987.

Your aftermarket heads should have enough valve travel to take 1.6 rockers on top of your stock cam (in essence just like putting a bigger cam in the motor). What you'll need to watch for is the pushrod clearance where the pushrods go through the head. Make sure there's no rubbing. You might have to change pushrod length too. Best way to find out, get an adjustable pushrod 1.6 rocker and check out what length pushrod gets you the right alignment of the rocker arm tip on the valve stem. The stock length might work, might not.

Go with heavywall pushrods when you get new ones.  

I went ahead and just made the order for stock rockers, pushrods and the cylinder heads (because the heads won't accept the 1.6's without machining for the push rods).  I understand everything you're saying, but my circumstances really aren't making it too easy... I'm at college right now and the only reason I'm doing any of this is because my head gasket blew, and instead of gambling on 23 year old heads that probably need some $$$ to make them reliable I got the cheapest performance heads I could.  On top of that its an old engine and I'm not going to go all out on it, just because I plan to later.  I'll be doing this job on a basketball court in the snow on top of that!!!