I ran a 250 in my last '49 and it worked great. I used the crossmembers you mentioned along with the stock radiator and heat gauge, although I had to use an adapter to get the bourbon tube to fit in the 250 head. The only problem was the 4.11 rear end. I was running about 3000 rpm at 50 mph. I put in a 2.73 rear end and it dropped to about 1800 rpm at 65 mph! Out on the highway I had plenty of power and speed to keep up with most of the V8 guys. Not only that, but it looks stock! I used the original 3 speed column shifter for my TH 350 tranny. Took a little rigging, but it worked. I think you'll like this set-up.

My truck: She may not be the prettiest girl at the dance, but she's all mine!