Hello BEX.
Unfortunately all the information that I could find was an article in the Winter 2000 journal of the Canadian Aviation Historical Society. It describes the conversion from B-24 to C-87, but contains no real information on what would affect the flying qualities enough to make the aircraft a dog.
http://www.cahs.com/public/CAHSJournalWinter2000.pdf
The author does mention that the control cables had to be rerouted through the top of the aircraft. This could, I suppose, account for a certain amount of control sloppiness. Also, I believe the B-24 wing was an early version of a high aspect ratio, laminar flow airfoil. That type of wing performs better at altitude and not as well down low where most of the cargo flights operated. If indeed the engines were equipped with only a single stage supercharger, (no high blower) I can see why the performance left a lot to be desired. Of course this is pure speculation on my part.
You could try airtanker.com for more info. Up until July or August, I believe Hawkins and Powers operated a couple of PB4Y's, the US Navy single tail version of the B-24, in a firebomber role. Sadly, they lost one in an inflight structural failure along with the crew, and the second one has been grounded.