1988 Ford F-150 heads

I'm in the process of rebuilding the top-end of my 1988 Ford F-150 4x4 XLT.  I am having the heads reworked, replacing the timing chain, lifters, and water pump.  Is it necessary to replace the head bolts, rockers, and push rods?  And if I'm replacing one fuel injector should I replace the others?  Addtionally, How important is the vaccum steel line that runs between both the heads at the back of the motor.  Whoever owned it before pulled the smog pump off and the Cat. I plan on using the truck as a weekend driver, pull a pop-up camper, and for hunting...I just need it to run good when I need it.   

You can re-use all of those things.  You should make sure the rods are all the same length, and some people replace the head bolts.  But again, you can re-use all of those things.
 
You can relpace one injector.  That is fine.
 
I'd keep the vacuum line on, too.

That vacuum line that runs across the back of the heads is called a head-to-head pipe and is part of the thermactor system.  I know everyone says that is costs you power to run thermactor systems, but it is also good for the air you breathe.  It will help the cats to burn hydrocarbons and it also gives the exhaust a longer life.  If you state does emission tests, then definetely put it on.