hi paul

i hate to suggest this
but have you thought about modern
tab grain film and developers that are designed for low grain and hi sharpness ?

while i have used tech pan ( years ago ) i used it for extreme contrast
and made 16x20 enlargements from a tiny portion of a 35mm negative with no grain
it was fun, but using a LF camera and a modern tab grain emulsion with the right developer
can yield nearly grainless results, even enlarged to the extreme you wish for.

it only photographers that stick their nose up to a print and say " i can see grain "
its kind of a disease, and i really don't understand it.

good luck !
john