I scored this Crate Engine casting # 10066036 to throw in my 87 base Firebird. Everything went well, until time to bolt up the torque converter. Now....., the Flexplate off the Ole 305 that came out didn't fit the output on shaft on the 350 crate engine that is going in. I know a guy who is knowledgeable of GM, and he gave me a Flexplate that matches the diameter of the flange on the Crank, and the six bolt holes line up on the back of the Crate Engine. I thought this was "all good" until i got the engine and the Tranny Bell all matched up and bolted together. I mean like where a sheet of notebokk paper could barley fit in between right? Underneath, there is still like a half inch between the Flexplate and the Torque Converter??? What Happen? What did I do wrong, or need to do Differently? Also, on the Output shaft on the 350 Crate Engine, there is this "counterbalance/flange" lookin thingy with the following #'s on it, 8342685. (either 2 or S, not sure which) The Flexplate Bolts to the center and slightly to one side sits this Flange Thingy??? I put "the Indented Humps on the Tranny side facing the rear of the car before the engine install and it still didn't quite add up?

 

(Your 87 had this one):
86 & newer 305, 350, LT1, LT4: 3.00" bolt circle pattern. Counterweighted for internal balance. Made in 153 tooth for 86-97 F-bodies and other passenger cars. A matching starter has bolts straight across. For trucks, they were made 168 tooth and starter bolts fit diagonally.

(Your crate engine needs this one):
85 and older 265, 283, 327, 302, 307, 350, 305: 3.58" bolt circle pattern. No counterweight for neutral balance. These too, were made 168 tooth and 153 for passenger cars; trucks had the 168. The above starter compatibility holds true.

Cliff notes: Buy 85 Camaro flexplate. Install. Done.

 

Have you tried pulling the convertor forward to match the flexplate after the trans is bolted up tight to the engine block ?

As you have found out there are 2 different style of SBC crank flanges depending on the year of the engine
Because the balance arrangements are different ,the crank bolt patterns are also different to prevent the wrong style flexplate being used

 

Sounds like you have the flex plate bolted to the crank backwards. Or possibly, the flex plate is bent. In which case, just go to the parts store and get a new one; they're pretty cheeeeep, and that motor isn't going to stress it enough to where you need anything beyond a stock replacement one regardless.

The dimpled places in the flex plate where the bolt holes are, should be toward the converter. There should be a gap of around 1/8" - ¼" before bolting it all up. If you know what the 2 notches on the converter look like, the "gap" should be no more than about half of their depth.