Hostler beat me to it as far as trying the speed loader goes. If you look at a variety of grips (factory and otherwise) you will see all manner of shapes on the left grip panel; high or low, thick or thin, dished or with a thumb rest, ad nauseum. Obviously, the closer the grip gets to the open cylinder, the less room you will have to maneuver a speed loader of any sort, and about the only dependable measurement I can think of is to try the actual speed loader with the actual grips in question... not a great answer I know, but probably the answer.
BTW, and strictly as an extra possibility, I found some speed loaders for a J frame 38 that are constructed like speed strips but hold the rounds in a circle like a “hard body” speed loader... you put the rounds in then pull the tab to release them. I haven’t used them yet, but I currently have a pair on order.
Froggie
PS The item I have on order is a Zeta6 Speed Loader (on the ‘Bay, of course!) I’m sure it will work on my service stocks, but I’m also hoping it will work with a pair if finger groove combats with a big speed loader cut out. HKS loaders sorta work, but they aren’t Speed Loaders!