Fords had flathead 6’s stock so you will to work with the lengths. A 300 will definitely need a notched fire wall. I’m thinking you could gain some room up front.
I know you want it cool, and it's a cool thought... but for the work of making a 300 work, it would be a lot easier to install a 302 HO with a lot more power.
There’s some threads here on that subject. But seems like no one wanted to do the mods to make it work.
I can't speak for a shoebox, but I was able to get a 300 to fit in my '51 F-1 without cutting the firewall. Rad had to go forward about 1 inch. And to those that say a 302 has more power can go pound sand.
The turbo may help just a little. I am actually a great fan of 300-6's, having had a couple, in a '95 F150 and a (believe it) Winnebago class "A" motorhome. I find the idea of adding a turbo intriguing.
According to this you have 30 inches in length from the back mounting surface of the block to the water pump pulley. The big rub on most swaps is the front sump. Scroll down to the Ford six http://www.how-to-build-hotrods.com/engine-dimensions.html
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