Thanks Mike. I looked at the 15501 ball that was in the bell housing and thought it looked good compared to the stock 403 housing sitting beside it, but didn't actually measure them. I installed the Lakewood 15500 Fork assuming it would fit but it does have a beefier web along the top and that's where it's striking the firewall.
No getting around the mid plate being installed now so I'll take your advice and try switching the ball and the fork to see what difference that makes. FWIW, I called High River and they have the ball in stock so I'll pull the bell later on today and work on that this weekend to see if I can gain some clearance in there.
If not it'll be a BFH moment.
Cheers