I just got back from a trip and had asked this exact question before I left. I absolutely feel your pain. I'm pretty big; I'm 6'2" and around a 44-46 inch waist and 48 to 50 inch chest and I'm just as proportioned in the hips. I really had no real problem at any of the parks.
At Kings Island the tightest rides are what you would expect. I was worried on Diamondback, but I was able to squeeze into it just fine. They say that all of the seats are the same, but I'm not sure that is the case. Rows 2, 4, and 6 I was able to get into without any problem. We decided to try the back and sat in row 14. I couldn't get the lap restraint down to where they wanted it on my own, and a ride-op had to give the bar a push to get it there. Firehawk was also definitely a bit more snug than the Superman rides I've ridden, but I got myself buckled just fine. The other two rides that seemed a little tight were Flight of Fear and Invertigo. Everything else was no problem that I can remember. You will absolutely LOVE Diamondback, btw.
I knew when we visited Cedar Point that Millennium and Top Thrill would be a no go for me, and I was right. After working up quite a sweat, I was able to get the test seatbelt buckled on Top Thrill, and the guy working the end of the line said that I should be able to make it, but when I got to the ride, I found the seat belt much tighter and had to take the walk of shame. Like I said though, I carry weight in the hip area which really hurts me on those rides; it seems like it's a proportion thing on both of them, so try out the test seats for sure. I also have to give five stars to two ride-ops who worked some serious magic by getting me into Wicked Twister. I had no problem with any of the flats like MaxAir, Sky Hawk, nor Power Tower.
At great Adventure I had no problem on Nitro, which is the greatest ride in the park. When we were getting on Bizarro my niece did give my restraint a shove so I could get it buckled, but I made it just fine. Batman was actually the tightest Batman ride I've ever ridden. The ride-op had to use some muscle to get me buckled, but again, I made it. As I expected, I did have trouble on El Toro. Again, I don't really have a protruding belly, so I'm sure it was the damn hips again...with my mouth getting some of the blame. I was able to get the seatbelt buckled but couldn't get the light for the lap bar to turn green. I probably could have made it with more ride-op magic, but the line was horrendous because in true Great Adventure fashion they were running it intermittently with only one train.
I've never been to Busch Gardens, so I can't help you there.
I hope this helps. If you have any questions about any specific rides, please post them. I'll keep checking back to help you out as much as I can.
I honestly don't think you'll have a problem. The main thing is just go and enjoy yourself!!