Why do you feel you need a big block Toploader? The small block ones are tough enough for big blocks, too. I ran one behind a 427 lowriser for years without breaking the input or output. Besides the inputs on most big block toploaders are too short to work with the small block bellhousings.
the 390 and the cj/boss toploaders are close ratio and the input is maybe 3/4 inch to short on the snout. if you have a big spline then the input can be extended and then you would have to get custom order clutch disc's. other than that, dan williams has a three peice input that can be used on big and small blocks and with close or wide ratio and can be had in small,big or 26 spline.
thats true there were wide ratio 390 toploaders. Ive seen a few but seems that most around my area were close ratio. I have yet to see a factory ford wide ratio big spline, but havent seen any of the factory custom drag or nascar trannies.
Go to 4speedToploaders.com, David Kee's site, he's got an ID # chart that list's all the different variations and another chart for the custom race Toploaders produced "back in the day"