What flexplate with Reid bell?
ATI makes a flex plate for a the Reid cased Th400. It uses a stock starter and it's $190. Part number ATI-915650X for 6 bolt crank.
That plate is the same as quicktime or Hayes RM-947. It may work with ATI bell for their 400 but, I doubt it. When you bolt a flat flexplate to a Mopar crank and bolt a Reid bell the starter ring is closer to the block than the oem gear that was made on the mopar converter. Therefore the starter when bolted directly to the bell the gear is meshed with starter ring without starter activation. There is a starter spacer that is 3/4" thick that would correct this.
The issue I am curious about if one used this flexplate and starter spacer you still have to pilot the gm converter. Gm is 1.u00". The crank pocket is 1.705". Chrysler is larger. ATI and other companies may have that part also. The RM-947 has a washer welded to the converter bolt holes obviously with spacing in mind. I seriously doubt it fits without some adjustments.
I ran into some of this with my R5 to Reid bell and made a part to simply solve all of the spacing issues. The other part of the fix was a 1/4" midplate to gain spacing. Most racers run one anyway. The midplate protects the block in the event of a converter explosion. Most cast bells with sfi 30.1 cert do not have a block plate. All quicktime bells have a 1/8" steel plate, they feel the spec requires. 1/4" aluminum also covers this.
I really need to finish mocking this up before coming to any more conclusions. I am 100% sure about the starter. As well the pilot. Just need to prove the converter to pump clearance will work with my idea.
As far as ATI400. They did their homework. It has both ATI and reid patterns. If you use a Reid bell, you simply tell ATI and they machine the lip smaller on the transmission and you use your Reid bell.