Maybe worth wipping one off and having a look, mine are 8.5” x 20 on the front and am running 245 x45, I think as @BognorMotors mentioned, 265 would be the best option, that’s if your running a rim of 9” or wider.I don't actually know to be honest - they were on the van from new. The original tyres were 275's, but 35's not 40's so it's the extra sidewall height that has made the difference.
Also is it a T32?, if so, load rating needs to be above 103.