[jim sewell]

Toby
There are several steps to take to determine what piston you are after.
1. measure the bore accurately , ie snap gauges , digital verniers etc
These engines came out as 3.500 inches ( blitz's )or 3. 5625 inches ( others )
and determine if bored out to o/size ( some piston manufactures stamp the o/s on the head of the piston.
2.Referring to the picture where you are hoisting out the engine and looking at the angle of the water pump inlet pipe I would say this is a blitz engine that originally had domed cast iron pistons and a single hole in the block where the water pump bolts up to.Check how many holes in your block .
3. Looking at the picture of the piston I would guess that it is made of aluminium alloy , so it may have been a replacement set at some time , maybe had a rebore at the same time.
4. Looking at the picture of the block with the pistons in it , they appear to be flat heads , so the replacement pistons may not have had domed heads.
Check whether they are flat or not .
Summary
Blitz engines originally had domed cast iron pistons , were 216 ci , had single hole from water pump to block . A similar engine made about the same time had two holes into the block with flat top pistons , not many engine parts are interchangeable between these two engines.
So firstly determine what engine you have.
Regards
Jim S.