Too much? Wont fit? Too big?

Partly depends on what vehicle you have (didn't tell). Early A's (62-66) have a narrower engine bay and different K-frame, but a few people have squeezed a big-block into those. There were a few factory big-block A's, so there are K-frames that work (67+ cars), though very rare so most use Schumacher motor brackets. Have you looked at the Big-Block forum? There is also a website www.bigblockdart.com or such.

From the little I recall, a 1978 big-block is not ideal. I think at some point, they changed to a cast-iron crankshaft and open chamber heads (lower compression). Unless you go major and run headers thru the inner fenders, most people say the big-block exhaust will be so restricted that a small-block will perform better, and actually let you negotiate a turn.