273 302 318 340 build

IMHO, I would not hesitate to go with the 1.88" valves if the price is the same IN THE smaller chamber heads. In low to mid range, I am sure it will translate into better cylinder filling. Remember, with low compression, not only is the compression lower, the amount of spent gases pushed out each exhaust stroke is less (due to more uncleared volume) and any spent gases left in the cylinder is like having an EGR system in the engine..... So anything you can do to help make it easier to clear and then refill the cylinder is gonna help. It is one of those things that contributes to overall efficiency. It is also going to make the heads respond better to any cam changes you make, both in cam specs and cam timing; the small size of the stock valves is one factor that 'gets in the way' to some extent of the cam controlling the air flow.

If you are having trouble with the idea of higher CR pistons, then forget the 3671587 heads. They are going to just mean more CR work to make it work for you in street use.

If they mention any option like a Serdi cut under the seats, I would do that too if not too pricey and in budget. But not all shops have that and it is not normally free!