Re: interchangeability between 390 and 460?
No, C6 transmissions are engine family specific as the bellhousing & trans are a one piece casting ie, small block C6 will only fit Cleveland or Windsor, FE (390 etc) will only fit FE, & 385 series (429, 460 etc) will only fit these engines (but will also suit 351M & 400M).
Power steer, AC, etc will require different brackets to fit 460. Tailshaft will require shortening and possibly a different size rear uni (depending on diff being used).
If your looking to unload the 390 stuff let me know.