First, I'd suggest clipping the sides together. You can use strands of yarn to tie the sides together, or what I like to use are little hair clips. Anything to hold it as you seam, making sure the bottom and top edges are even.
Then, as you seam, you can skip a row here and there on the longer side as you work your way up, tugging on the sweater to keep it from getting tight. It should work out fine.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry