Recommended run in for new pistons only
 

..

1000 miles, change gear regular, don't go over 4000rpm much and don't let it strain in any gear.

As above moderate driving with a little boost every now and again. Every builder has their own opinion on this, I was recommended a couple of hundred miles using a mineral oil (synthetic provides too much protection for rings to bed in correctly) drop the oil and replace with quality oil with a new filter :)

Crank and bearings all the same and no machine work to block or heads i should have added .

why only change the pistons?

Cracked ringlands

If you have dropped new piston/rings into original untreated bores there basically is nothing to bed into, as a basic requirement they need a glaze buster in them to give the ring a chance to bed into the bore finish.

I would have thought cross hatching of the bores and new rings would have been beneficial while the pistons are out?

Holy cow, who advised you just to do the pistons?