So, I need that adapter ring and move the bolt holes, correct?

 

Yes that is the later flexplate which will work with the adapter ring. With the converter holes you have to check them before fitting the trans. Flip the converter over on it's snout and put the flexplate over it as it would be bolted up in the car. Depending which converter you have there are differences. With that plate you will probably have to drill new holes for the converter.

 

Thanks alot man, I appreciate it!

 

Back to the OP's original question: best choice transmission is subjective, 4L60, 4L80, Th400, all viable options for a street/strip car.
You mentioned the 400 so some of the parts needed for that swap: crossmember(we sell BMR), cooler, crank adapter(or custom converter for dished flexplate).
The transmission will bolt up no issues you just won't use the 12'oclock bolt hole.
If you plan to race it, especially more than street driving, then i would definitely go with a higher stall converter, if you are street driving it you can still do a more modest stall converter that will offer a balance of both.

Look, its no longer politically correct to call them tranny's, we now must call them gender neutral ratio reduction boxes. And the fluid is now gender neutral shift juice.

 

Will a Buick th400 converter fit my flexplate?

 

There is nothing wrong with a 4l60. It pulls my Yukon XL fine. Why give up overdrive for a turbo 400?

 

To avoid the hassle/aggravation/expense of the 4l60.