will 99 heads go on a 98 block since the 98's have perimeter bolt heads

 

yes. But you will have to buy new valve covers and the brackets for the coil packs to mount.

 

i picked up my centerbolt valvecovers and coil pack brackets off of ebay.

 

New GM stuff is'nt that bad if you can't find used.
list for the brkts is 14.42 ea
list for the cvrs are 62.11 & 105.60
+ gskts & bolts
of course, you don't pay list....

 

Does a bear poop in the woods? Of course you can use the later model heads. But they flow the same, so if you are in the market for some heads check out Patriots stuff. The 98 heads are cheaper if I recall correctly. It's not like the different valve covers cause major oil leakage or anything.