Nathan, the quick way to check is to hold the shutter in front of you and rotate it slowly until the rear tube just starts to occlude the lens' exit pupil. Estimate or measure the angle the shutter's been rotated. Do the exercise with the lens wide open and at your likely shooting aperture. Stopping down will help a little, not enough.

David's right, an Ilex 3 truncated like mine will give you around 270 mm before mechanical vignetting bites. A truncated Alphax #3 should just do it for you.

David, I use a similar triangles model to predict mechanical vignetting with front-mounted lenses. (tube ID/tube length) * lens focal length = diameter of the circle that can be covered without mechanical vignetting. If, that is, the lens has that much coverage.