F250 rear spring upgrade to F350 or F450???

[injection direction]

I was changing a flat rear tire on drivers side and I noticed a BROKEN rear leaf spring!!!!! First of all, I do a lot of towing and hauling with my truck and I was wondering if this is a major issue that needs to be addressed now or if I can nurse this for a while or what??? Second, I would like to lower the rear end an inch or two because I have to raise my goosneck hitch to clear the back of the bed and then get out again and lower the gooseneck again to get down to the ball so I would like to get the rear down about two inches or so with whatever I do back there. There is currently a stock 4" block under the springs. I have and early 99 and I was thinking of adding overloads with bumpers on top like an F350 or even a 450. My question is can I remove the block and bolt on directly a set of F350 or 450 springs to get my ride hieght down a couple inches and increase the spring strength so that is wont sag too much under load. Sorry so long winded....any help would be great.

[Dave]

No you cant do that with a factory spring. However look into spring upgrades from deaver