Where you put them depends on whether you're having nearside or offside park blades (should be obvious from the wiper arms). This will determine which side of the scuttle the motor will be...
You can try and work out the angle of sweep from the number stamped onto the back of the crank wheel but it depends on how many teeth your rocker box wheel has. You'll usually get the standard teeth number but there are a few rockers out there with fewer teeth (or is it more- whichever, it mucks up your sweep!!). I'd recommend temporarily attaching a rocker box to the motor and manually checking the sweep. Use this to determine roughly where the offside box needs to go. Even if it's not 100% where you wanted it'll be close enough- its not as if the offside blade needs to be that accurate as field-of-view is less important. Once that's in place, you can measure the distance between the beginning of the offside sweep and where you want the drivers sweep to start and therefore work out the gap needed between rocker boxes....
Clear as mud?
I didn't do it this way initially and had some holes to fill and gelcoat afterwards from failed attempts
