JBL,
Thanks for the bump on the mailbox...cleaned house a little.
.250" x 48" square steel weighs 164#, less the hole, plus the fitting. I gotta guess that you are pushing 200# p/m. Very unwieldy. The fitting should be galvanized and I don't know how it will react to the heat of a weld. That, and steel rusts so you will want coat the plate with epoxy paint. By the time you are done, you may find that the $125-$150 (del'd, approx) per PVC unit is not a bad decision.
...another thought...the PVC plate "gives". As you backfill, if the soil is replaced and compacted a little more in front or a little less in back, the plate flexes. If this occurs with a very solid steel unit, you may impart undue stresses and twist that will transmit and manifest at the coupling.
Not tryin' to be negative...tryin' to identify the worst case scenario so you can make a good decision.
What are your plans for the actual pipe?