Dick gave you the summary of the configurations info for these two destroyers. Which configuration you do for each depends on WHEN in their careers you model them. I can't comment on which one you "should" build, that is your decision. In the interest of clarity and sorry, some redundancy ...
MEREDITH (DD-434) a GLEAVES class unit from the first series (prior to DD-444).
She completed (April 1941 after commissioning on 1 March 1941) in the then standard initial configuration of five 5-in, 10-TT, and six 50-cal MGs, but soon was modified to the Summer 1941 King Board configuration with four 5-in guns, 10-TT and twelve 50-cal.
Then in early 1942 either before or after she transitioned to the Pacific with her DesDiv 22 sisters (DD-433, 434, 435, and 436) and USS HORNET, some of her 50-cal MGs were replaced with 20-mm guns ... likely four. Records are fuzzy, but this is what is seen in surviving photos for DesDiv 22 destroyers.
As Dick said in July 1942 MEREDITH was upgraded to the temporary approved armament for the DesDiv 22 destroyers with nine 20-mm guns ... and the GRAYSON photos at Mare Island are the best available for this configuration. GRAYSON was modified at MINY, whereas the other three sisters were modified at Pearl Harbor, but likely followed the same plans. MEREDITH was lost in this configuration. There is a DML 1/700 scale kit of this configuration.
AARON WARD (DD-483) a GLEAVES class unit from the repeat batch (DD-453 onwards).
She was completed in the standard configuration for the earliest of these units, four 5-in, one quad 1.1-in mount in the starboard "tub" of the two "tubs" intended for twin 40-mm mounts once they were available (a 20-mm was in the portside tub), five 20-mm guns, and 5-TT. And yes the bridge was modified somewhat as well.
In late 1942 AARON WARD was upgraded at Pearl Harbor with two twin 40-mm mounts (replacing the quad 1.1-in mount), seven 20-mm guns, and 5-TT. Her version of this upgrade is different than her sisters modified stateside. You would need to study photos to understand. No plans of this version are available as far as I know.
Now then as for your question about the length of the aft deckhouse, I have not studied the plans for the BENSON-GLEAVES class comparing the first batch to the repeat batch in detail. You didn't provide whether the two plans "scaled" out a 3-ft difference or if the plans provided dimensions. Most plans available showing the overall ship, don't give dimensions like for the overall deckhouse length. But they do show the frame numbers which for this class would be 21-in apart.
I have very limited number of plans for the BENSON-GLEAVES class destroyers. But here is a main deck view for MAYO, a first batch BENSON class unit that should have the same size aft deckhouse. And below that is a profile view, I don't have a deck view of this ship, that shows the length of the aft deckhouse. Both show that the aft deckhouse length is roughly frames 129 to 163 ... which should be roughly 59.5-ft (actual could be 60-ft or 59-ft to the outsides fore and aft of the deckhouse). My drawings are from Booklet of General Plans at the National Archives, and these overall "Assembly" drawings may not be dead-on. There is a vent (and sometimes cabinets) that appears against the aft bulkhead on some drawings at different periods in time that could account for the length difference depending how the show-up on the plans you have
On this page at HNSA, there are plans for two late version BENSON-GLEAVES units ... DD-616 NEILDS (Benson class 1942) and DD-618 DAVISON as a DMS conversion (GLEAVES class 1958) ...
http://www.hnsa.org/doc/plans/index.htm ... they are different from either MEREDITH or AARON WARD (late production repeat BENSON-GLEAVES units), but may prove useful. Both of these plans also show the aft deckhouse sits between frames 129 and 163.
