I'm 6' 3" used to be about 400, first time I went to DL at that weight was April 2018, then I could ride everything in the park except Space Mountain, and even then I could get in the ride but it felt tight, turns out I didn't know of the leg cross method which changed everything.
since then I am down about 60 lb and have visited DL on 2 separate 10 day trips (Oct 2018/Feb 2019)
I can fit on everything, leg cross method makes Space Mountain a breeze now.
Also I am sure you know pull the seatbelt out first and buckle it before sitting it on any ride (Indy, Star Tours, Guardians,matterhorn)
For Radiator Springs for some reason people say to sit in the back, no, sit in the front FOR SURE.
As for matterhorn, only sit in the front, and if you arent tall you will have no issues, I used to HATE matterhorn, even crossing my legs my knees still were even with vehicle opening and it banged my knees up SO BAD i couldn't walk comfortably after.
This past trip I discovered if you are tall and you do cross your legs you can slide your body down a bit and your knees can get inside the vehicle comfortably and then you have a bunch more room vs sitting up straight with your legs against your chest.
It's really hard to explain in text.
Either way hope that helps