The FP-336 fits with a couple of caveats. The screws that came with it were too large, but luckily I had some 6-48 x 1/8 oval head in the parts cabinet. Perfect size and the oval head self-centers the sight in the mount holes.
The second issue is the contours of the receiver. The bevel on either side of the Marlin 336 are apparently quite a bit more pronounced than on the RG4570. It keeps the elevation from bottoming out properly, but should not be an issue in the field. Won't know until I get it out to the range.