Since all your blacks should not (if we;re duplicating the original look) all have the same look or finish, if your going with rattle cans you will want to have a group of acceptable products in your arsenal
Take a panel of some kind (I typically use a gang plate used for framing with out the teeth just thew holes) and spray bands of each product in bands about 1-2" wide the length of the plate. Remember to label which product with number (since providers can change or up date products) on the face or back side.
You can also use this to compare these finishes with original ones when your dissembling and record them if you choose. Know Charles did the same years ago (test panel for comparison) I also did the same with about 10 different engine blues that is around here somewhere
An additional step that seems to change the glossiness of the products and helps provide a nicer IMHO finish in many cases is baking the part as they sometimes did. Seems to make the finish last longer since it has no catalyze in the product. But this will require some testing to see what rattle can paint will take the heat and which ones can not.
Haven't done this for years but recall it was something like 250 degrees for 5-8 minutes - approx. Of course the thickness of the metal part guided towards how long it baked
For specifics I've never found a Eastwood product where I liked their blacks. But I've not tried using rattle cans for engine compartments nor large area for over a decade. When I did it had too much brown in the final look for me.
Another "trick" is to carefully heat the cans before their use to help the flow better and some products have better flowing nozzles while others will plug quickly so make sure the can gets well shook before use
