I have seen conflicting reviews on whether the AX15 or the Nv3550 is the 'stronger' transmission. They seem very comparable in design, gear ratio, factory applications. is it that they are to comparable to differentiate or is one or the other a little stronger, looking for opinion. I have a 375 ft-400 ft# 351W planned for my 68, which wont see much offroad use.
Hard to say...
The AX-15 doesn't seem to have the GVWR rating spec'd like the NV3550, but the general consensus is they're similar in strength due to the application. Many will lean toward the NV3550 because the ratings are actually posted, 300 ft/lbs torque @ 7,200 lbs GVW
The only issue with the NV3550 is that the transmission is no longer in production, where as the AX-15 can still be purchased new. I found a much better deal on an AX-15 locally because the transmission was more readily available. This is the only reason why I chose the AX-15 over the NV3550. I only have a few thousand miles on the AX-15, but no issues to report (351W, 4.88 gears, 35" tires).
For a built or stroked 351W, I would go with a truck transmission, Ford ZF or NV4500. Both the NV3550 & AX15 are designed for 6 cyl. engines and will have limitations. I think we get away with using these transmissions successfully in the Bronco because of the lower GVWR.