here is a link with some frame dimensions from the blazer. the thread is talking about converting a 4x4 to a 2wd but one member posted frame drawings for both. maybe that will give you an idea of where to install the driveline. you can see the blazer had quite a drop in the frame behind the firewall area and the 55-59 trucks are basically flat frames. a large drop in the cross member would be needed, like you show in your pics. personally, I would fab a new cross member once you get the engine/trans/transfer case location figured out. remember that the driveshaft will need some room to expand or contract in length as the suspension travels up and down over bumps. I know some 4x4 guys that have lost a driveshaft due to that when the slip joint expands past it's limits-lifted trucks, gettin air.
hope the link helps. maybe a private message to the poster would get results. if you click on the name of the poster another box will come up with options to choose from. pick private message and go from there.the thread is from 2014 so he may have moved on but he may also still have the info.
http://67-72chevytrucks.com/vboard/s...d.php?t=619850