Converter too close to housing?

I do a lot of bench jobs for beginners to install and the double clack or thump method is a flirtation with disaster. I finally posted "Automatic Transmission Installation Instructions" on CL Inland Empire, Ca. In it I say to turn the converter so that one of the lugs is at the top. Then place a straightedge across the front and measure back to one of the side pads. Then place the straightedge across the back of the flex plate and measure to the block. The trans measurement should be at least 1/8' more than the motor measurement. Also to loosen the distributor cap and screw 2 bolts into the block that have had their heads cut off and ground to a point, or 2 screwed into the bottom trans holes that go thru the block from the front.