Hot Rodding is all about exploring and building alternatives to what is everybody else is doing. It also started out as (and for some of us still is) an attempt to have a car at a performance level comparable to or better than what we could go out and buy and doing it at a lesser cost.

If your looking at sub-framing the car as an attempt to have something a little different than someone else, go for it. Look for something with a front tread width very close to what you have and unless you are a very good welder, find someone who is and has experience with this type of conversion. Talk to him about what needs to be done and go from there.

If the main reason your exploring sub-frames is because you've priced the different options available from the vendors and figure this would be a cheaper (and possibly easier) I think you will find out differently. Unless you have all or most of the equipment, room (about 2 bays worth), time and most importantly experience to do this yourself, farming it out will be far more expensive than buying bolt on upgrades and result in an equal performance level.

I have done sub frame conversions, weld-in suspension systems, and bolt on upgrades. Due to the time and cost involved the first 2 are generally reserved for vehicles that have severely outdated suspension designs (straight axels, and some of the early IFS) or parts are not readily available or adaptable for.

The 55-57 Chevys had a relatively decent front suspension design to begin with and as Joe said there are a bunch of parts available for them to upgrade them to contempory standards (based on the original frame and suspension design) with more coming out all the time. Chances are if there was a cheap and easy subframe swap for these cars you would see a lot of them installed already.