The trick is to get the axle centered in the M105 bed wheel well.
Test fitting the M105 bed on the frame is a good idea, before drilling and cutting.
If you use the bumperettes you’ll have to trim the rear-most M105 bed mount to make it all fit.
Bumperettes or not, make sure the tailgate opens all the way before you decide where to cut the frame. Make sure to take in account the rubber “bump stops” on the tailgate, they will change your measurements by an inch or so.
The rear axle location will determine your driveshaft length, typically the stock driveshaft will need to be lengthened about 5”.