C-135 C-153 switch

But since the 504 is an update to the 340 there is a chance. From the parts books and other's posts, I see that there are at least 2 flavors of C135's and 2 flavors of C153's depending on what they were installed in.

I've read that the C153's from the 504 had about a 2" wider bellhousing pattern than the C153 from a baler/swather..., which was the same as the C123 pattern. The C135 came in a 404 as well as baler/swather type installations which I would expect to have the shorter bellhousing pattern, but a C135 is also in the 340 (Farmall) which I would expect to have the wider pattern, and possibly fit the 504. The 330/340 came in a Utility model as well and I don't know if those motors were cast differently or not (I don't remember if they used frame rails or the block as an integral frame).

So, it depends on what the C135 came out of. If you don't know, I would measure the distance across the bellhousing bolt pattern on the C135 and compare it to your 504.
Stan(VA).