Well, I'm not going to try and convince you of anything...I will tell you my opinion, and then you can do whatever you like
1) Straight 30W motor oil does not contain the amount of additives a multi-grade does, notability anti-wear and film thickness modifiers. This actually provides more base oil by volume in a bottle.
2) Everything I have read says to use a straight 30W on older era motors (like the 7M). It allows wear to take place to properly seat the rings. Newer (especially high performance) motors come from the factory with a syn oil fill...these engines have closer tolerances than those of the past. You have to decide what you want to go with.
3) Straight 30W is very good at dissolving assembly lube.
4) It's cheap...you're going to do a couple oil changes in the 1st 1,500 miles. Why use anything else. While your at it, grab a couple SuperTech filters at WalMart...pretty good filter, especially for as long as they will be on the car.
Castrol 30 HD is an API SM service oil...it well exceeds the oils originally spec'ed for the 7M. It will be fine.