The issue with the GAs is that their g/box cross member bolts inboard of the chassis rails.
The GBs bolt up through the rails.
So if you can find a GB cross member, you can drill through the rails, add crush tubes and bolt up
Welding to rails is forbidden where I'm from, so you cant just extend the GA inboard mounts rearward

Also you may have to move the gear-stick hole rearward a bit.
Just cut away from the back of the hole and weld it to the front

Some 5 speeds have shorter extension housings than others - short is good.
Others here have more experience than me in this, so I'm happy to be corrected on any of this.
Cheers
T.