New convertible for wide-shouldered baby

Hi, my youngest is 6 months (16lbs, don't know how long, but in 6 month clothes). He's a preemie, born at 31 weeks, so although a nice size and with fairly good head control, he's nowhere near sitting up (not even rolling over yet). He's in a chicco keyfit 30, slightly above second harness slots, and we're running into a problem with his shoulders being too wide for the seat. They roll forward and are starting to be squished in there. He also screams nonstop in his seat. We drive an 07 Sedona with high mileage (so will likely replace in about 2 years if we can make it last). My other two are 5.5 yrs (46", 42lbs) in a Frontier CT, and 2.5 yrs (34", 33lbs) RF in a Boulevard CT. For ease of use, I'd love to get another boulevard CT, but we're considering the clek fllo. Not considering dionos 'cause we've had them and hated them. When we replace this van, we're hoping to get into a midsize SUV (would love to have them three across for convenience). So two questions: Internal shoulder width of boulevard versus fllo versus foonf (on this site, boulevard has the narrowest, but I'm wondering if that's an error maybe?)? And, given that the two older kids' seats are so ginormous anyway, does getting a fllo for the youngest make any sense in terms of hoping for a three across? Or do the size of the other two seats preclude that anyway? Anything else I should consider for this hopefully-final carseat purchase?

Width will vary by where you measure on the boulevard. The clek seats are pretty narrow in my experience.

Have you considered other seats at all? For instance Evenflo makes some wider interiored seats...

I wonder if maybe a Graco Milestone would be a good choice in this situation. It looks like it may be 2" wider internally than the Boulevard, but not sure where those measurements are being taken on both seats. Plus you have the added bonus of HBB capability down the road.

My 3 year old fits great in his Milestone. He doesn't have particularly wide shoulders, but he has plenty of space.

Thanks, good to know about the cleks tending to narrow. I'll have a look at the milestone. Am I pretty much hooped on the three across possibility with two big seats and one wide-shouldered boy?

I think you'd probably have to replace at least the Frontier.