I've got a 58mm Motta, there is a tiny bit of wiggle room in the basket...
Using a Mahlkonig Vario, not getting clumps AFAICT but I haven't broken the grind up in the basket after I dose for ages... It looks pretty good and unclumpy... I tap the coffee level before tamping...
until I changed my shower plate I used to get really inconsistent flow through mine which led to a lot of channeling/spurters - the IMS precision screen helped a lot. still get the occasional messy shot which I can't nail down but that also helped a lot.
Switching to a convex tamper helped a lot for me. 58.4 as well not 58mm.
Are you breaking up any clumps in your grind? What's your grinder?