It is an interesting and plausible hypothesis. However, I would view it as babies at "risk" of SIDS or ASD (i.e. susceptibility). It isn't preordained by any means.

it was just a curiosity, since my first child died of sids, and my youngest has autism

SIDS declined by half from 1 in 1000 to 0.5 in 1000. However, during the same time ASD has sharply increased 1 in 50, which is much faster rise that the rate of decline in SIDS and significantly higher (and if I recall correctly, the UC Davis study found only a third of the increase in ASD cases are due to better diagnosis). However, regardless of this weak correlation, your hypothesis could very well be true that a baby with 10x ASD risk would also have 10x SIDS risk, though this has not been studied. For example, low birth weight baby has significantly more risk for ASD as well as SIDS as shown in studies (and dozens of other medical conditions).

i was just wondering because of the decrease in sids happened about the same time as the rise in autism.

The only common link between the two that I am aware of is both being "reported" as adverse effect of certain vaccines (specifically DTAP vaccines e.g. Tripedia DTAP actually disclosed in their package insert). There is also a likely genetic susceptibility component in both. On whether SIDS victims would have been autistic, though plausible if there is genetic susceptibility component, one cannot make such a correlation given SIDS incident rate rate was and is still fairly low (0.5 or 1.5 in 1000) which is now where close to autism prevalence (1 in 50).