A 383 stroke is 3-3/8". A 340 stroke is 3-5/16". A difference of only 1/16" or 1/32" on the radius. I kinda look at a 383 as a bigger 340. If it's in good shape I wouldn't be worried to 6000 rpm.
But as already stated, running some documented test runs is your best bet. You might consider a blow proof bell housing as you go forward.
