as one who has done this, you need two "pucks" that fit the end bearing bores, [roughly .003-.005 smaller than the bearing bores] as well as two pucks fitting a bare center section bearing caps, tightened to specs, installed into the housing you are narrowing.
the pucks need a center bore of 2.003-2.005 for a straight piece of 2" solid bar to pass through to act as a centering fixture. this bar needs to be about 12" longer than the overall length of the finished housing, so you have 6" or so sticking out past the bearing ends on each side.
bolt the center section into the housing. [after you have narrowed it to your liking]
slide the alignment bar through the housing so an equal amount is protruding from each end.
put the bearing end pucks into the bearing ends, then slide the ends/puck assembly onto the alignment bar. tack each end in place, [four places, 90 degrees apart, allowing each to cool, then do the opposite side tack, which [if done correctly] will still allow the alignment bar to turn freely in the puck bores. take your time, doing one end at a time. doing it this way will allow you use the center section pucks as datum, and if for some reason the housing gets distorted and the bar will not pas through the bearing puck, that end can be heated and quenched or put in a shop press to straighten that tube so the alignment bar will pass freely through the bearing end puck after final welding is done. rinse and repeat for the other side.
this is just a quick synopsis of the process, but gives an idea how this is done.
i realize not just anyone will be able to accomplish this in their home shop, due to not having a lathe and chunks of stock to make the required pucks, but just what i have done in the past. your mileage will vary.