The V8 in the 57 was mounted back against the firewall and has the radiator mounted back about 6 inches with it. If you are swapping a V8 into a 55 then your radiator will be way up front away from the V8 fan. That is why some sites sell a Fan housing for just that case (plastic). The housing I found out will not work on an original V8 only on the V8 into a 6 install where there is space for the housing. I still have mine it is useless to me will not fit a original V8 truck.
So be prepared to install a fan housing/enclosure or have a really long fan extension like 9 inches long? or go with electric fan. The V8 will overheat otherwise. Mine overheats anytime I am stopped for 5 mintes it gets up to the M or MP in "T E M P" on the guage. When it gets to M or past M it is cooking. Normal cruse temp it is on E of the TEMP guage. As long as I am going 20 or so it never overheats. stop for a light and it starts to warm. Stop for 10 minutes and your cooking. And that is with the FAN blade 3/4 -1 inch from the radiator. Yours will be 6 inches away or more as your 6 cylinder is longer and the radiator is mounted closer to the bumper.
The original V8 installs have a deck spacer and the radiator is mounted 6 inches or so further back than a stock 6 cylinder.
Here are some pistures from when I tried to install a fan enclouser. It would not fit since I would have to cut 3 to 4 or more inches out of it and then the round hole would become square since the housing is shaped that way.
fan enclosure work See how the radiator is mounted 6 inches back from a 6 cylinder one in this picture. on a 6 there is no flat deck extension in front.
deck extension pushes radiator back 
