The problem is not gonna be fittin' the motor, it's all the other stuff that has to be upgraded: cooling, steering, suspension, brakes, rear diff, etc., etc.

If I were gonna do it, I'd be lookin' for a derelict 66-69 Fairlane, Comet or Falcon to use as a donor car. I'd cut the front and rear frame thirds off the '49 and weld the Fairlane/Comet/Falcon front and rear subframes onto the middle part of the '49 frame. That way, you'd be set up for motor mounts, trans mount, radiator mount (using the donor core support), newer power steering and suspension and brakes, rear suspension and differential, etc. You could also set the body to the height and attitude you wanted it at without having to fosdick around with cut springs/dropped spindles, etc. and the attendant evil handling that comes with messin' up the suspension geometry. A motorless donor that came with an FE would be a plus, giving you the good diff. Headers might be difficult to come by with power steering, but I'm thinkin' a set of Sanderson shorties would work. Crites sells a plan to convert to disc brakes on the front for $25. It uses OEM stuff that you can find in the boneyards.
http://www.critesrestoration.com/fairlane.htm

For the skinny on the myriad of other stuff involved, I'd get one or more engine swap books and bring myself up to speed.
http://www.amazon.com/Engine-Swappi...niques-Shop/dp/1855200171/ref=pd_bbs_sr_1?ie=UTF8&s=books&qid=1203034763&sr=8-1
http://www.amazon.com/Complete-Book...-Swapping-Storer/dp/0822700913/ref=sr_1_2?ie=UTF8&s=books&qid=1203034834&sr=8-2
http://www.amazon.com/Practical-Eng...ing-John-Thawley/dp/0936834110/ref=sr_1_3?ie=UTF8&s=books&qid=1203034834&sr=8-3

 

I have seen a 460 in one so a 390 should fit,I would recomend installing something like a fatmans front clip to make it easier to install the motor. The original suspension is made so the motor has to have a center sump oil pan and making one for a FE would be difficult,you might have to use the 6 cylinder radiator brackets to move it in front of the core support for more room. Jeff

 

My 50' has a 302 in it and it is a tight fit! I don't know who installed it, but it works!