Do you have an oversized aftermarket oil pan or a stock replacement center sump style?
On mine I have a 360 with a Milodon 7 quart oil pan. There was not enough room to get the engine and transmission in without getting stuck between the k-frame and the trans tunnel. I had to separate mine and do the engine from the top and then the trans from the bottom. Then once they were in place I have to lift the engine back up while connected to the transmission to get the left side header in place from the bottom.
I have smaller tubes than yours because I am using Doug's headers.
This would be a good time to start taking everything out the bottom on the k-frame. For me too next time, maybe. Although I am used to separating them.