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GM flex plates often have dual bolt spacing (6 bolt holes) to fit a torque converter with either a 11.50" and 10.75" bolt pad spacing. The TH350 and TH400 torque converters come with 3 bolt pads but may have either bolt spacings. If your flex plate only has 3 bolt holes and they don't fit the bolt spacing on your new torque converter, you will have to purchase a flexplate that has a dual bolt circle.

I went through this puzzle with my '91 S10 Blazer 4.3L V6 engine when I had the engine built with 350 Chevy forged pistons and internally balanced. The 4L60 (700-R4) transmission in my S10 Blazer use counter-weighted flex plates for externally balanced engines. When I had the engine built and internally balanced, I had to use a heavy duty (0.125" thick) 168 tooth flex-plate, 3" flange bolt circle for the one-piec e seal without a counter weight and was zero blanced.

My engine builder knew exactly which HD flex plate to use. He provided an ATI #915534 non-counter weighted dual pattern flex-plate to use with a torque converter that has either a 10.75" and a 11.50" bolt circle, 168 tooth starter gear, a 3" crank flange bolt circle and zero blanced.