If you've already reassembled the arms (bushings in place) I'd pass, considering the heat the bushings would be exposed to. Also, the rubber bushings will do some deformation by design, and any effort to stiffen the arm is probably not productive.
(editing)--after re-reading your post, I think you are talking more about putting a plate across the whole bottom of the arm. On this type of vehicle, with limited grip and mostly operating on smooth surfaces, I don't think there is anything to gain. The added weight of the reinforcing is counterproductive, as you are adding unsprung weight, unless you compensate by making something else on the unsprung side lighter--brakes and/or wheels being the obvious. Truth be known, the sprung/unsprung ratio on these cars probably would have to change considerably before a real difference would be detectable. I'll add here that I am unaware of any history of failure of the B-body arms in the area you are talking about reinforcing, and I really doubt you would be able to do any A-B-A test to prove that the effort did anything, other than give you some peace of mind and lighten your wallet. Rather than go to the trouble, let's see what the promised tubular front arms that are in the pipeline look like--spend your money there rather than reinforcing the stock arm(s)--just my opinion.
(the following is speaking only to adding a reinforcement to the ends of the open channels on the inboard end(s) of the arms, next to the bushings)
The arms I've built (for quite a few B-body owners) normally get the reinforcement in conjunction with Global West DelALum bushings. The aluminum shell of the GW bushings, being softer than the steel shell of OE-style rubber bushings, will benefit from the added stability of the weld reinforcement in the bushing boss area, to reduce any tendency for flex or "walk" in the bushing area that might tend to "chew" on the aluminum. Since the DelALum bushing also has "zero" compliance, impact forces that normally are dissipated by rubber bushing compression now pass into the arms, making the reinforcement more desireable, though NOT essential. Unless someone asked NOT to have it done, all of the arms I've built with GW bushings have had the reinforcement.
The B-body is a big/heavy car, and with higher spring rates in popular use, lowering/crashing bumps stops, large wheels and tires, and sprited driving (pick your poisons here), it is a good idea to do this, but with rubber, the inherent compliance lessens the need to do the extra (weld) work.