Automatic column shifter jumps down a gear on initial acceleration

[Accelo]

My truck is a 1970 Converted to a 4L60E behind a LS1. I used a aftermarket column shifter rod to clear the headers.
My automatic column shifter jumps down a gear on initial acceleration. Typically moves from OD to Drive. It does not jump out of gear; the shifter just moves down one gear. I have checked and I don't have a broken motor mount. I have also inspected all the bolts for torque. This includes the transmission mounting bracket.

I have always wondered why GM used the complicated linkage from the column to the transmission. (see attached picture of stock linkage) The single rod seems so elegant. However, I know GM didn't add parts just for fun. There has been a lot of discussion on how to make the automatic linkage work but very little on why it was built this way. Now, I am guessing, the stock linkage uses a bell crank from frame rail to trans for this exact reason.

As soon as I can get some help, I will start engine and hold my foot on the brake pedal. Then drop it in drive and goose the throttle while holding the brakes hard. While someone watches to see if the engine moves when you going from park to drive.

Maybe I will find my issue? I am going to look hard at reinstalling the factory style brackets. The factory stuff seems to get rid of the Micky Mouse feeling of a user modified ride.

For some unknown reason the transmission will not shift down correctly unless the selector is in OD. It works great in drive. Another reason I need to get this straightened out. I need to experiment with this to confirm. preliminary observations is the WOT shift points also change when the transmission is in any gear but OD? Weird.
Cheers