microscopic photography with Pentax 67

Yes.

One of the older tricks is to simply set up a tripod so that the lens looks in the eyepiece of the microscope without touching and place an dark cloth or piece of pipe insulating foam between the two.

There are several good microphotography sites on the web. Nikon has a site, Olympus has a site and Micorscopy UK is a good site.

I have just gotten started and am using 35mm and 4x5

The trick is not getting a picture it is getting a picture worth having. To do that you get into special lighting.

See the nikon or olympus site.

Something to shoot for:

http://www.microscopyu.com/smallworld/gallery/contests/2005/index2005.html

http://www.microscopy-uk.org.uk/mag/artmay05/swgallery1.html

I've done microscopy with a Pentax 67 and 35mm. I had a Pentax lens reversing ring welded onto a T-ring adapter. This setup worked fine but I often found softness at the edge of the film - probably due to the ocular not being designed to project a flat field over such a large surface area.

Instead, I'd recommend getting a modest digital SLR and creating mosaiced images that are as large as but better in quality than if one had used 4x5. Because the digital sensor is smaller the image quality is likely to be higher than if spreadout over a larger area. By taking 20 images and putting them together in a computer, I think the overall quality will be greater than 4x5.