Did you trim the top material, as in its width, or did you trim the padding off from under the the original width of the material?
David Coco it an expert on upholstery, so shortly, he should chime in to offer advice.
However, if you cannot make the material stretch from side to side, with or without padding beneath, you likely need to start over with new stock.
Oh well, it is just another few hundred $$$! ;-(
Further, do you have the drip rail yet?
If you do not have one in your pile of parts, you should have one made to install, as it will act as the facia, under which the top material should reach, but that may have to wait.
Your Series 80, DeLuxe Sedan is Grand and will be better after you work on it for a few hundred hours.
I cannot tell you how many hours / days I spent lying on my back on a creeper staring at the underside of my Series 80, while I tried to figure out how it worked, what it was missing and cleaning it up.
That was all before installing the many eBay find missing parts.
Your car will be great, it will just take some time, so be patient and do your best to only repair / replace things once or twice. HA!
I hope to meet you in Pennsylvania.