You really do need the crotch straps fitted to prevent submarine gold.. Unsure whether you can use 3”, however several manufacturers do sell 3” HANS compliant straps. I know mine are HANS compliant from Simon at Meteor and are 3” shoulder straps. Holes- there is guidance online for spacing with Hans and angle behind the seat. I can’t recall recommended spacing but some guidance here https://www.tillett.co.uk/shop/documents/downloads/seat-belt-fitting-information-2012.pdf And, I do have a very sexy Stand 21 Ultimate carbon Hans available size L, 20 degree, only as personally i really didn’t get on with it..