Roy's basic patterns are very nice. His instructions for assembly are "Swell Chest" instead of drape. Crooked cut instead of straight. There are reasons for both.
The original reasons for drape was to create an image that the skimpy guy did not appear skimpy. Many developments happened after that as the imagination wandered to many places. With thousands of tailors and customers competing against each other is going to create thousands of variations. Some tailors are very narrow minded. I was told, back in the sixties, that the best drape is down the center of the chest. Some tailors like it in front of the scye (armhole).
The Swell method allows for the artistry of the Iron. The best of these is art top art. Absolute magnificence.
When making, for a guide, words like elegance, magnificent, a collection of these kinds of words can be infused into the garment.
In the old days there wasn't much paper. My grandad, was asked how to make a pattern, that this young man found in a book. The pattern was about four inches (10 cm) in size. It had two measurements. One of the symbols is T and I don't remember the other one. But,the T might mean thumb width. The other measurement is put your arm straight out in front of you. Put your thumb up. The distance from the top of your thumb to the tip of your nose is the other measurement. Everyone has different lengths. But, you learn to work with yours for making garments for everyone. There is a simple way to enlarge the pattern to working sizes. And then, all the adjustments for the customer, including styles, fashions, whatever. The reason why I'm saying this is because I want your mind flexible. I don't have the books and literature available to help. Some of the old tailors said that the shoulder point at neck is the key to the pattern. It can be used that way very well. Perhaps average chest size is 40", waist 35", seat 42". Differences are 5" and 2" from chest. If you are going to use Roy's pattern, use his differences. I'm going to suggest using your waist measurement and use Roy's differences for the chest, perhaps the seat measure. When making the pattern pay attention to the ease additions and where they are at. You might need a couple extras. One tailor said for large muscle guys to take two measurements of the chest. Relaxed and, under pressure. Add half the difference to the relaxed, and use this chest number. Arnold Schwarzenegger went to a tailor. The tailor didn't know how to make clothes for such large muscles. When finished Arnold put it on and flexed his muscles. The coat ripped to shreds. Most these strong guys take their coats off if going to lift something heavy. Generally, how much expansion room is necessary? And when relaxed don't want the garment baggy. Distributing the cloth to hide bagginess can be figured at the fitting.
Another consideration. Breast dart going clear down to the hem to get rid of extra cloth below the chest. A group of Italian tailors cut all their coats that way. Some others only for large chest. Some tailors never run that dart to the chest. Make the pattern for proportionate 37 inch waist, and we'll think about the chest adjustments later. Chest measure will probably be 42-43.
It is also good to look at inlays for coat patterns to see where chest expansion will be going. What I'm thinking is that the neck point and front edge of syce are most important. Everything else can be widen or narrower. In other words, the chest can be wider and the underarm moved over. And that the warp threads hang perfectly plum where they are supposed to. The block pattern can be slipped right and left to accomplish for larger chest.
I'll give you some measurements for help later.