[Pete]

The one that will do the most good is the heaviest you can find and adapt.
This may work best but it is not always the best solution for people that do not want to modify the frame.
Most ones that are big enough to do some good require trimming the front crossmember.
The amount trimmed does not weaken the crossmember and done properly will hardly be noticable.

I have been using the stock 8 inch diameter one from a big block Chev. Easy to modify.

Once you have one installed and drive around for a half hour or so and then feel it, if it is warm it is working. If it is cold, you probably don't need one.