I think nightmare is an understatment when installing the SLP headers. The install blows ***. There is only ONE way to get them up there. I would love to help you but the headers stressed us out to a point of drinking beers every few minutes during the install and by the time it actually butted up to the block and went in correctly...well...I don't remember.
The best advice I can give is to mess with it untill fustrated and take a break from it. Getting pissed will only make it that much harder.
take the bolt out of the motormount.
Then jack the engine up on an angle.
You will have to play with the height of the motor...going up and down a few times...it will go in just make sure you have one other person other than yourself.