[1968CHEVYC10]

The transmission is still in the truck? This is what I would do, remove the transmission, bolt the bellhousing the the motor then put the motor in, and install the transmission after that. The bolts holding the trans to the bellhousing are easier to get to (and there's less of them) than the bellhousing bolts. That way you wont have to mess with line stuff up when you put the motor in.