Unfamilia
The "minor" diference of 3mm represents an increase of about 4.1% in bore size. By way of comparison, many of the late type V8 engines are limited to a MAXIMUM overbore of .040" or about 1mm. In a 4" bore this represents about 1%, yet the little A12 will go even more.
Your friend should get an understanding of what a "minor" oversize is.

As for the "minor" 7mm increase in stroke, this looks like a 10% increase to me & i don't know what he calls "major"

By using a 77mm bore & the A14 crank, it increases the capacity by about 23% with completely stock parts.

The 383 cube Holden is an increase of 24%, but requires costly special parts, so he has no problem pushing Holden engines out by 24% with expensive special parts, but doing something similar to a Datsun with inexpensive stock parts seems to be a no no.

If he can nomimate a supplier of Datsun pistons of the required 76mm bore with the smaller wrist pin, i think we would all love to know of them. Methinks that they would need to be made by a specialist piston manufacturer at great cost.

My Dodge hemi has a stroke of almost 4" & these engines, & when suitably "tuned" will rev quite happily to about 6,500rpm without problems, & the A14 will do the same, so how high does he think that your A12 will go?
For street use, anything much over 6,000rpm is really not necessary & the 77mm crank will handle that with ease.

As for the rods, just how hard does he think it is for a competent machine shop to bore the small end in 4 con rods. It seems that he thinks it is preferable to have custom pistons made. If push comes to shove, i wonder if one of our US members could provide some A12a rods as these have the correct length & pin size.

Is he being lazy? Thats not for me to say, but the fact that you asked..............