The problem with using a 1.60 exhaust valve in the 235/261 is that the valve shrouding will get much worse and you will need to remove enough chamber to cause at least a 1/2 point drop in compression. If you are going to use forced induction or custom pistons, no big deal, but do the head first before ordering pistons so you can get an acurrate volume measurement on the "new" chambers.
Considering the potencial costs of significant head work as well as custom pistons, you may want to concider turbocharging. It would prove to be more bang for the buck, in my opinion. 300 hp should be doable on pump gas, but you'll never get close normally aspirated.
