Joe, I would think a 2.5" system with a cross over would be just right, since the 141 heads are a little better than the 140's and you should gain some with a better breathing exhaust system.

http://www.pypesexhaust.com/ is said to have a great bolt on/weld up system with an X cross over etc... I've heard many say it went on just as it should, and fit with little or no tweaking.

Gardner and others make a good direct replacement system too...

You should be able to clamp the whole system, and then have it welded later.

Brett - 67 Drop Top 455!