G`day ,
as it doesn`t need to be exact .
If you put a rod down a plug hole and rotate the engine through a full rotation , you should get an idea .
71mm is a std 3.5 a stroker will have around 86mm
The stroker will be second oversize , 20 thou but you can`t find that without dismantling .
The Australian flapper ECU lack what the UK ECU has , an enrichment at full throttle but i doubt cause for your situation .

