I have Downey rar springs on my truck, they are stock length. They are definately heavd duty, very stiff and ride like crap. I think the 51" would be a little more flexy simply because they are longer.
Why not go with 63" chevy springs? They will be cheaper and have been proven to work very well on toyotas. I have them on the back of my wheeling truck and they flex very well. They also ride better the stock toyota leaf springs. I think you will regret gong with a 51" Downey rear spring especially when you could have gone with the chevy 63".
