If you've got an electric stove, use that to solder. This way is much more simple. Just place the block on the stove and heat it up. Put the solder wherever it seems necessary. I've used this method several times with success.
Also make sure that whatever you're trying to heat your block on isn't soaking up some of the heat, I've tried to solder blocks sitting on my vice, and it's a no go, I usually try to put it on wood or something when I solder my blocks now. Goop works good to seal blocks too, but I just trust solder more I suppose.
I've found that 2mm copper plate I've used quite often in block builds shows no sign of distortion when heated, even if it's heated unevenly. I can get away with 1.5mm too if its not an overly large piece, but 1mm does warp or misshape easily.
This is using electrical solder with either a "blue Flame" or regular plumbing blow lamp with butane/propane mix, If you are using welding gas temps you certainly could get it too hot, and possibly cause warping.
I trust soldering more than any other block sealing method myself, but it is a skill you need to practise to learn how to do. Once you get the knack its very easy, simple and quick to do.
I won't post it in the thread because there is a lot of detail so have a read HERE, as I've soldered large copper blocks together using the method in the text. About the only thing I've changed is using the wood and I now use a small marble slab that was a scrap piece from a stone masons. The Marble can be warmed up a little before you start and once it's heated up by the blow lamp on the copper pieces it won't keep sucking it away like a metal vice. You should practise with some off cuts until you become proficient.
Another thing I've learned is you must have an escape hole, (the block can't be fully sealed, but needs a barb hole or smaller pilot hole if you can't position the barbs until it's soldered). If not the hot gases will expand in the sealed block insides and escape as it cools possibly creating small weep channels in the solder joint.
If none of that helps....... repost with more specifics like squeakygeek says