I have taken apart two 1970 W series motors and both had dome-top style pistons. Is the dome top still available as "new" or are all that are left are flat ones ?
I think the domes were rated at 8.2:1 compression ratio. The flat ones that I've seen are rated slightly less.
If the original pistons themselves are in good shape what would you do ? Re-ring them or buy new flat stock with less compression ? Where are members gone for new cylinders ? Surely those old oxidized ones can't dissipate heat that effectively.

All 1.7 Porsche had domed pistons and were 8:2:1. Flat top pistons where in the VW bus and VW 411. Do not know what is offered now. Every now and then a set of the OEM domed pistons show up for sale on Ebay or theSamba.com.

If they are within wear limit and no scuffing, do a light hone and re-ring. I did that to a GB code 2.0 with 8.5's and put on another 100K miles.