For all the money you would put into buying 99+ heads, intake, ect.. I think it would be cheaper and just as effective power wise to have your heads sent out to high quality machine shop and have them ported.
This is what I had to do when I swapped over two years ago:
For the fuel system I used my original 96 rails, had the mounting brackets cut off and re-welded on so they would bolt up to a 99+ manifold.
Other stuff you will need: a 99+ intake manifold and tube, 99+ throttle body and cruise control cables, exhaust manifolds, a chip or tuner burned to turn off the IMRCs, modify the vacuum lines, lenghten some of engine wiring harness wires, a set of 99+ valve covers(you can run 96-98s but the fittment around the throttle body is very snug), a 99+ coolant crossover tube with an extra bung welded into to run the ECT and temp sender, and you may possibly need a new midpipe, my 96-98 bbk o/r H did not fit the 99+ exhaust manifolds so I got a 99+ VRS o/r X.