The MGA seals the top of the dipstick tube with a LARGE (much larger than necessary) rubber boot that is a tight fit on the stick. You put the boot part way up on the stick, and then push the stick into place until it hits the bottom of the pan. In the process the boot lands on top of the tube and is pushed up the stick to exactly the right position. Next time you put the stick back into the engine the boot just touches the top of the tube at the same time that the stick hits the bottom of the pan.

Just a short note here. If you don't like the smell of hot oil splashing around your engine bay, do install the proper rubber top seal on the dipstick, and never run the engine with the dipstick removed. Rotation of the crankshaft throws oil with a vengeance directly at the bottom end of the dipstick tube. The slightest internal pressure and a little vent flow will throw out a lot of oil there if the dipstick tube is left open. Simply leaving the seal off of the dipstick can sometimes allow a small leak.