Before you go making spacers, you need to check to make sure the engine has all 4 spacers under it. Perhaps the rear ones are missing.
#2 in the pic:
1811 em.jpg
The QA42A will go on an 1811 with no issues.
No spacer washers needed. Your engine has to not be mounted correctly, or the pulley is wrong. You did change the pulley, correct? The one that it uses for the 1250 will not work with the necessary 5/8" belt the 1811 uses.
I've ran a QA42A on my 1811 one year when the 450 broke and I was waiting for parts. Mounted right up, no issues.
https://www.youtube.com/watch?v=gqK5r10xKBc&t=75s