Thanks!  I’m enjoying myself with them~

That depends on the head, most of the time.  I usually cut off the prongs and most of the long peg in the neck, so the head is just on the lowest cushion-shaped bit of the knob.  Some heads are good to go at that point, but for the oldest vintage heads I’ve swapped onto Made to Move bodies, I’ve needed to trim out the ring inside of the head a bit to make them sit down on the neck enough.  Otherwise you end up with towering giraffe neck because modern heads are made with a concave bit to facilitate head tilting and such.  It’s often the bits with the identifying dates and what not on vintage heads you have to scrape off.  I use an X-acto knife and just slowly peel out little slivers of vinyl from the neck hole.  

You can get heads on without modifying the neck peg at all, but I find it just makes adjustments more troublesome than it needs to be and especially with vintage heads you run higher risks of spitting vinyl if you leave all that fiddly business inside the head.  The modern heads are larger - I’m sure you’ve noticed - so sometimes after rerooting or repairing or whatever you’re up to with a vintage head, the long peg in the modern dolls is too long for the old heads anyhow and makes it hard to get over the whole peg without stress.  (I know you’ve got a Fashion Queen, so hair is not an issue, but just saying generally!)