Enginequest heads
http://www.eqcylinderheads.com/performance.html#chevy1

Manufactured in Queensland, Australia by Toowoomba Metal Technologies.
Newly designed to be much more efficient than previous versions, and are not allowed to be cast until flow numbers and Dyno numbers exceed previously PUBLISHED numbers, averaging 10 - 15 CFM greater than the other manufactures.
Cast at an ISO 9001 facility where one head from the beginning of each run is digitally checked for all tolerances, and each head is checked prior to being shipped to ensure EQ’s number one concern, maximum quality. Gone are the days of “if it looks like a cylinder head, stick it in a box”.
Manufactured with a .4375 thick head deck, enabling better heat transfer, and allowing for a better machining platform for decking.
EQ Cylinder Heads is the only company to manufacture a LOW RAIL valve cover. Our early GM head has a machined valve cover rail that enables the installer to run the head in an early GM vehicle where valve cover clearance is an issue.
Currently using ONLY phosphorous bronze guides in the performance head line.
Using a DURA BOND 30000 series exhaust seat in both the stock replacement and the performance cylinder heads.
A TRUE Vortec cylinder head! Unlike the competition’s cylinder head, where a late model bolt pattern is applied to an early head, the EQ Vortec uses the latest in Vortec technology in our combustion chamber. End Result? More power.
Priced competitively to all other cast iron cylinder heads.
Sold through traditional engine parts distributors and all sales are driven through our distributors. We do not sell to your customer, we refer them back to you!
EQ Cylinder Head is the only head maker in the business offering a full 1year parts and labor warranty on stock replacement applications. (See catalog for warranty details).
some flow numbers: http://www.rogersperformance.com/EQFlow.htm

I used the XM 270 in my 406 build-up a few years ago, and it idles very smooth. I don't think you'll have any problem with the 270. I also have the merc silent choice diverters so I do run through the y-pipe at idle around the docks and haven't had any trouble with reversion, etc. If I were going to do it again I'd probably push the cam specs a little farther out there, or ideally install a roller cam.

hay mate , you are right on the money with them heads ,they are superb. if you are getting the heads from over here why don,t you contact clive stenlake at wade cams and get the best cam money will buy as well. cheers tassie boy