For no reason other than the size of the vehicles they came in, I'd say the GM/Jeep Saginaw.... it's pretty easy to port, requires disassembly to do it right.. which is a MESSY job, and have a manual handy.
The Toyota IFS boxes are much easier to port, and are actually really good boxes. However, replacement parts for the Saginaw boxes are more readily available and cheaper.
If you run into any problems, we can provide parts or technical help.
now you've got me curious: the Toyota boxes are vertical, aren't they? (my Google button is busted)... if so, how strong are they???
the reason I ask is they may solve a problem I might run into on a big heavy ambulance deal. Say... 8k loaded up. W/hydro assist and mild, mild wheeling, would a Toyota box hold up???