Where did you get the 420k post-conversion for the Westin? If it will be 60k new points per night then 7 nights will cost 360k because you get every 5th night free with points. As 360k MR = 120k SPG points nothing changes for the Westin.
The Delta now costs 40k/night, so 240k for 7 nights with the 5th night free and will go down to 35k, so 210k for 7 nights.

If you want the Delta you're better off waiting until the 18th to book, if you want the Westin then it doesn't matter.

You could also go the travel package route and buy a category 8 TP with 120k miles for 360k MR points if you transfer 77k SPG points. This would allow you to book the Delta today and attach the category 8 (40k per night) certificate.
However, as the Delta is dropping to 35k/night you could also take a bit of a risk and go for a category 7 (35k/night) TP with 120k miles and wait to attach the certificate. The risk here lies in the fact that Marriott has not yet disclosed what exactly will happen with unattached certificates. We know that we won't be able to attach them to a reservation until September 18th, but we don't know what value or category will be assigned to them in the new system. In theory your certificate for a current cat 7 would be equal in value to a new cat 5 but no one knows how they'll handle this in practice so there's a bit of a risk with this approach. But it would save you 30k MR compared to buying a cat 8 and attaching it now.