You need to give the 335 series engine forum, there is a guy who built a 400, and has injected it, NO they aren't a big block and don't weigh near as much either, far better to build because of it. The modifieds have a taller deck, but other than that, they are still the size of a Cleveland, so will drop in where a Windsor came out withaout heavy loss of room like a 460. I beat a friends built 302 with a stock 400 in a T-bird, supprised me greatly, ugly tank would go.
You can use the 90's 460 heads and FI system to make the older block fuel injected, you definately want the newer heads, the 71 will likely have sodium filled exhaust valves, designed for regular leaded gas.