I can't say for sure, but the 3.0 was discontinued right when the Mazda gear box was introduced to MMC in the first place. Maybe it's a little too much torque from the V6 3.0. The 3.7 would certainly have overtaxed the MX5 NC gear box.
I know that Griffo of this parish was slightly concerned of torque handling when he performed his Ecoboost transplant to a Plus 4 (with MX5 box) several years ago now, especially when he wound up the boost to give, I believe, 260 lbft!
I believe vehicle weight may play a part with gearbox torque handling, and a Classic Morgan is always going to be a little less than any MX5. I was told that the ZF gearbox fitted to the 351 big valve Ford V8 in a Cobra kit I was building would be fine as excess torque would simply be spun away by smoking tyres
