Are you sure you are not accounting for too much wood being left after assembly? Or maybe a stock not inlet for a barrel that wide?
If it's a Colerain O/R 20ga. it is only 1.062" at the breach as it is and an 1" isn't but a hair's breadth.
Now, if you have the Griffin Fowler barrel it is available 20ga. in 1.125" and 1.230" and the latter shouldn't even be a problem as it is less than an 1/8" wider per side.
J.D.K.