I haven't messed with 273 very much but it seems your engine has been messed with some so it's hard to say what you have now. 273 had very small valves from factory but not knowing what heads you have changes that. You would have to mill the heads and/or use thinner head gaskets to get the compression up. If you mill too much you will have to get shorter pushrods. I doubt you'll see much gain by gasket matching unless the mismatch is way off. Have you thought about upgrading your heads to a set of 308 heads? If your planning on building the 360 you would be money ahead finding and rebuilding a better head that you could use on both engines.
Also there is always porting your own heads. Most people frown on home porting but I've learned alot by doing it and have become decent at doing it. Porting takes alot of time and not much money and I find it fun to do expescially watching the meter go up while my heads are getting flow tested.