I have had a couple transmissions shipped, bought on E-bay.
One went UPS, the other went FED-EX, just depended on what the seller was used to using.
The trans was about 90 pounds, the shipping was reasonable. That might be the upper end of what is practical for easy shipping.
Might try to get the weight of each box as small as possible, how much can you take off the engine? Can you ship some of it separately; flywheel, head, any smaller parts that could be removed and sent in a different box.
I had 4 cast iron heads sent, boxed one at a time, all 4 at once would have been a problem.
Of course get them drained and as dry as possible then seal them up as much as practical so they don't drip.
If you want to ship all of it together, then maybe attach to a pallet and use a bigger shipping company, that might require shipping to a business, not a house.