this is not for heads, but go to the boneyard and find a timing gear set from a 460 in a two-ton truck, like a farm truck,or a two ton flat. put that sucker on that engine and it'll be all you need. trust me, it works! 0-80 in about five seconds.
Do a good bowl blending on both ports and be sure to clean out the obstructions in the exhaust port. I dont recommend enlargening the runner sizes any. Just clean them up,and be sure to get a good smooth transition from the runner to the bowl. Also get a good 3 angle valve job; picks up a lot of flow numbers on its own. Stock cam is good for over 5000 RPM with this port job, and headers. If nothing else, get the heads also cut about .030" to bump up your compression if this is a smog engine. The smoggers only run about 8:1 compression. Getting up around 9- 9.5:1 will help your engine emencely and it wont beat the bottom end out. These mods on a stock engine should net you about 45-55 peak hp. and a noticeable increase all the way through the power band. The engine should be good for an easy 5500 RPM.
Get an earlier timing chain and gears for a pre smog engine. (pre 72). This will advance your cam quite a bit and restore a lot of power lost by the retarding for smog purposes. Most performance timing sets are set to the earlier speck, but I would also recommend checking the timing on the cam and set it to recommended factory specs for the early Limas.