The 620 mounts are 3/4" shorter vertically than the 240sx mounts.
The 521 mounts are 3/8" taller vertically than the 240sx mounts.
I'll get a side by side picture tonight.
Both of my mount sets use 620 L20B isolators.
Another thing I hope to be working on soon is SR mounts for both trucks. I have someone sending me stock SR mounts and willing to test fit them in his 521.
Reading back through and realized I never took comparison pics.
Here are the KA mounts
Passenger side stock 240sx, 620, 521 - Driver stock 240sx, 620, 521
Passenger side stock 240sx, 620, 521
Driver stock 240sx, 620, 521
The mount I have pictured is the early 521 transmission rubber mount and the cross member mounting bracket. Be sure to use all 4 transmission mount bracket mounting holes on the cross member.
I would change the transmission mount to the newer type and build the rear mount to fit. This is Anchor Part # 2742 or DEA Part# A2742
The reason I suggest this mount is it will fit 4 speeds, 5 speeds and automatics.
Fits this range of trucks.
NISSAN 521 PICKUP 1972
NISSAN 620 PICKUP 1972-1979
NISSAN 720 PICKUP 1980-1986
The Anchor and DEA mounts are from 72 to 86. I know the 620 used this mount but have not seen it used stock on a 521
You have just have a 5-speed swapped in?
Can you take some measurements for me? I'd like to see if my mount would work for just a 5-speed swap behind the stock L series.