Doug,
In my 500? sprint motor I am using pistons from a Kawasaki Z400 twin. They are 64mm, so take the motor up to about 530cc (ideal for those who want to make use of worn out barrels), and the pin height is OK (just needs a new small end bush as the pin is 15mm dia). The top is domed but gives about the same compression ratio as the standard DT5 petrol pistons.
Regarding the Arias pistons - from what Henry told me, the first batch they made for him were too heavy, so he sent them back to be modified. The second batch were fine and giving good results in his motors (12.18secs with 112mph terminal for the standing start 1/4 mile).
Hope this is of some help,
Regards,
Eddie.