I used an old skool fitter that came highly recommended, and hes done a lot of classic work for big restoration firms like JD Classics, as Colin says, can be messy, but he makes it look so easy and all left clean, the sealant he uses dosnt go hard just like ford sealant didn't, on mine, ford stuff was still pliable when I cleaned the rubbers before he re fitted them, and that's after 45 years, fords was like a white putty, but modern is black
don't forget, its not just there to stop the water getting in and keeping you dry, its there to stop that water creating corrosion between the seal and body and carpet and floor after its leaked down there
if you look at a pattern window seal, they never seat round the corners properly and the first place they split