The right place is where the light rays come together which is probably in the middle of the lens. Made a simple aperture and try both locations. To get the effective aperture, try meter the image on the ground glass (assuming you have one) with and without the aperture making sure you are metering only the ground glass and the same area each time. Then meter the scene assuming f5.3 and multiply the exposure by the ratio of the meter readings on the ground glass. Disclaimer: I've never read this or tried it, but I'm pretty sure it would work!
Personally, I'd just get some ND film to pull down the exposure to something reasonable and enjoy the wide open look. Stopping down in the wrong place will probably hurt more than help.