It probably will fit, but the Sigma 28-300mm f/3.5-6.3 DG MACRO is not one of the lenses they suggest you use the converters on. As it is a budget lens with quite an ambitious range, it is probably fully stretched already and you might get results hardly any better with the 1.4 converter than without it and just more heavily cropped.

You can get an answer direct from [email protected] (and more lens info from that web-site) or maybe someone else will have more direct experience.

Do you already have the body and if so what other lenses already?