hmmm....asking for an epi in advance ...
i think that you can bear in mind that scans and estimations of head size and baby size can be a bit hit and miss.....
i would look at managing your diet and things like that to see if you can avoid piles , if that is possible
lost of water, fruit, fibre, if you need to take iron supplement, that can make you constipated, so mabye look at something like spatone for the last trimester, which will boost your iron levels, without constipating you
also, your undercarriage has been strecthced once, so might not take as much of a battering second time
you might find that all fours or a supported squat is a good position for delivering a big headed baby..
or in water to support the perineum
and as Drmcmoo has said, don;t try to hold back on the pushing
you might find that you are able to get away with a smaller tear or less of an episiotomy
the really important thing is not to stress in advance and during the birth that you will need an epi , or have a big tear, you need to accept baby is coming out , big head or not, and you need to relaaaaaaaax all the perineal area , rather than tense as he is born
sheila kitzinger and janet balaskas have done some interesting writing about pre birth relaxation and breathing to practice , to keep all your undercarriage nice and erm, untense!!