
Like Tom Smith pointed out, McCulloch yellow had variations from the 50's to the 70's. I tried to share these variations with different Original parts and newly painted parts in digital pictures which don't show much difference other than shine from new paint. The point is from what I see is they are all very similar.
Yes I agree 100% McCulloch didn't use primer and I don't want anyone to think they must do things the way way I did. I'm really just trying to help by providing more options and sharing some fun about our hobby.
I used primer and called this post a "Party" as it's not as serious as restoring a million dollar Hemi Cuda or COPO Camaro. I'm sure you could get serious about restoring a mac to the level of obtaining a pint of original touchup paint, but then you'd have to ask if it was the same mix used during the production year of your motor and on and on.
My intent in this paint procedure was different than what you might think

Many of the parts should have been tossed out, but with the price of MC10 shrouds lately I couldn't see wasting a part that could be used for a "display only" engine in my collection. Pit holes in some of the pieces went all the way thru the metal and looked like swiss cheese. I used JB weld with a metal backing on the most severe areas and then plastic body filler for scratches and dents. All parts got a acid bath (PPG metal conditioners) and epoxy primer before any patchwork began as a method to prevent corrosion in the future. After all the patch work was done I was left with some minor pits and sanding scratches which were smoothed-out with a coat of Primer Surfacer then wet sanding with 400 paper.
The McCulloch factory wasn't faced with what we have to work with today where we want to preserve the metal and keep our engines looking good for shows. Where I live raw Mac metal discolors and in some cases grows a white dusty type of corrosion; thats the reason everything got painted inside and out. I'm sick of opening a point box finding white powder. Maybe in your area you don't have this problem.
I could keep going on and on about reconditioning metal and don't want to bore you. My website has a "John T" special links page on techniques.
Here's a pic during the priming process where you can see the layers of the darker Epoxy Primer and lighter Primer surfacer in the rough areas and some raw parts after metal conditioner.