[Safari Larry]

Pontiac heads are interchangeable side to side but 1957-59 tubes are not. Looking at the tubes from the end, they are mirror images of each other (see attachments). The tube can be inserted in the head only from one end and the flared end needs to be in the front. If you moved a left head with left tube to the right side then the flare would be at the back of head which would be incorrect.

The 1955-56 tube can be inserted in either end of the head. If you moved a left head to the right side then you would need to remove the tube and insert it from the other direction -- so the flare is in front.

I'll take a look at my 56 heads today, didn't get a chance to yesterday, sometimes life gets in the way of important stuff.