Hi,
The only force acting on the pump body is from the v belt, and as it's
a rotational force, plus a bit of tension, it's not particularly high. Provided the bolts, threads,
mounting surfaces and the pulley bearing are all in good condition,
I don't see an issue.
Although the pump may well take larger bolts, the block is already compromised
by the threads, or some of them, entering the water jacket. Enlarging those
may well lead to unintended consequences!
Yes, you're quite right. I'm sure Lancia sized the bolts adequately. They did use M8 studs on the water pump to back rail flange, so it was clearly a conscious decision to use M6 bolts on the pump to block connection.
Graham