Not impossible, but hard.
Then you'll still need to assemble the viable viral particles. This will probably require the creation of artificial chromosomes needed for the viral replication, and then innoculating human cell culture with them, alongside with the synthetic viral DNA.
This is a level that requires years of work from a major biolab.
So the tools are there.
However on the other hand - making something the right mix of lethal, but still able to spread I suspect is incredibly hard - if it was easy we'd all be dead already ( from viruses etc naturally evolving ).
Sorry for being pedantic.
That was in 1978! I believe we didn't have biosafety levels back then in the UK.
Edit: at least the BSL levels came 6 years later, but the UK has different names for those
>Over the next two decades, growing CDC, NIH, and OSHA participation in ABSA annual meetings further solidified biosafety guidelines, culminating in the 1984 publication of the first edition of the text, Biosafety in Microbiological and Biomedical Laboratories (BMBL). The BMBL guidelines laid out four levels of increasingly intensive safety practices, equipment, facilities, and engineering controls to be employed in the safe handling of microbial agents: Biosafety Levels 1, 2, 3, and 4 (BSL-1, -2, -3, and -4),
Edit: I can't reply, so I'll say that an extinct horsepox virus was recreated from sequence, the same procedure in theory should work on smallpox