Correct. If you just shove wire wheel axles into a steel wheel housing, they will engage, but will have 7/8" less engagement on each side - a recipe for broken axles. Do it right.
If you really want to avoid using the correct housing, you CAN use a Triumph adaptor bolted to the MGA steel wheel hubs. It will result in a wider track width, but nonetheless within wheel wells.
You will need to either cut down the studs so they don't hit the wire wheel and bottom, or better yet, make alloy spacers under the adaptors so you can leave the studs alone. You also need to buy the special nuts used on the Triumphs with those adaptors. See a picture of how I did the front hubs on my Jamaican. Also, bear in mind that MGA and TR have 7/16" stds but MGB uses 1/2", so if you had an early MGB axle under there, you'd need to change out the studs.