the double row timing chain will swap straight over. check the condition of everything, you may or may not need a new chain and tensioner (the double row chains last for ages). biggest pain is having to drop the sump to pull the timing cover. :faq: if you do that, consider adding a reground cam while you have it apart. i "think" you can pull the lifters without having to remove the head (depending on sludge buildup), with a 3/4k anyway.
make sure you remove the little studs under the timing cover and replace with bolts.... will make it much easier if you ever pull the timing cover again, as you only need to drop the sump half as much.
with your heads: is the 3k one a bigport, with the welschplug on the back? if it has the plate with 4 bolts on the back of it, it will be pretty much identical to a 4k head. either way, a 3k head will give a bit more compression on a 4k.