Buy in experience from USA in product pricing where they were world leaders then you can make a profit on models that you can sell in quantity.
You need to start with the formation of BMC and fast forward to British Leyland. In other words get the economies of scale as soon as possible. Lets Rootes do the same.
Thirdly ruthlessly rationalise the groups components. So many minor parts can be shared around. Don't let them fight for the staus quo, make them fight for a good place in the new vision.
Personally, using the period technology, I would use the Daimler V8 for the engine base. A 1.2 litre 4, 2.0 litre V8, 3 litre V8 and 4.5 litre V8 plus a good 2.5 diesel 4, possibly bought in or on a Daimler type block.
Teach the component suppliers (especially the Prince of Darkness) that you will buy elsewhere if they don't meet your standards. They need you more than you need them. Customers will judge you by their performance. For them the Lucas alternator didn't fail, the Morris didn't charge it's battery.
Emphasise reliability and quality to the engineers and staff. Everything must work and fit. Use incentives and pride to commit staff to these goals. This cuts across everything. Set acheivable high standards and insist they are met and give staff the tools to be able to reach them. They are professional qualified car makers not assembly line drudges. The minimum standard has to be that everything works in all weathers and continues to do so until appropriate mileages are reached. In return demonstrate to the staff that they are valued. Training and qualifications. Clean company uniforms across all staff. Spend money on cleaning and painting, quality canteens and toilets etc. Get you product pricing people to do a cost price base company car workers can buy. Austin company car parks used to be full of Fords and Morris Marinas. If you can then get company based unions rather than trade based ones. Work with them to improve staff standards and self value. A good union is your ally in developing staff and identifying staff issues before they cause disruption.
Weather the storm of existing pride in the old badge companies. They have to be transferred to image blocs. e.g. Triumph for small sporting quality saloons, e.g. MG for small sports cars, Jaguar for larger ones, Rover for larger sub jaguar saloons, Jaguar for the large ones. Austin for bread and butter saloons. Alvis, Riley, Wolseley, Vanden Plas etc. can be reserved for the super deluxe ends of each range. e.g. The Rover saloons have a 2.0 litre 4 engine but there is a top end with the 3.0 litre V8 as an Alvis.So you have a single name for each image and a single image for each name but the mechanics are group mechanics.
The dealers will squeal so back those that will come on board and encourage them to expand into the territory of those who won't. Give them an undertaking that you are committed to do away with the warranty repairs otherwise so common and make servicing and repairs easier so that they can do more work with less staff. Treat them as part of the company and supply them with the latest tools to do the work.
Borrow in the market place not from government.
Let your experimental staff play with new technology but stick with known arrangements until the reliability and servicing costs can be proven to be up to standard. So no FWD until it can compete on merit.
Minimise your base floorpans as well as gearboxes and engines so RWD means a small gearbox, a medium/diesel one and a large one. Buy in automatic boxes and make this a V8 standard. Same for differentials and axles, brakes and steering racks.
Now you are in a position to seek larger markets overseas with dealers and buyers confident in the products. Remember in OTL the japanese products only sold on price and reliability. Poor handling, ride and braking etc. but they kept on going and you could afford them. You will be asking a modest premium but giving a finish quality, reliability and image that they cannot.
The light goods vehicle side has to be seen as equal in importance and the same principles apply entirely with reliability and cheap servicing being even more important and can be identified with the Morris name in a light car based van, small true van, medium true van and a true LGV for light quasi lorry work.
The heavy commercial lorry/bus side would not fit well with the car/light goods range and needs it's own separate group but the above principles still hold.
OK, nice model but how do you get there? Do so while you can still sell what you have by channelling investment into a certain area and demonstrating it works before seeking new investment from the market on a proven business plan. Having the model you can eliminate (painfully) investment in areas where they will not contribute to the plan in the future.
I would begin with the light goods range. In the 1960's this will call for the two 4 cylinder types and the diesel together with the small and medium gearboxes etc. If quality and reliability can be achieved in this demanding high mileage market then it will perform in cars.
I definitely would not do a pick and mix of all the possible prototypes etc. that OTL might have considered. This ATL needs it's own.
I see a small sports car floorpan with 1.2 Austin mechanicals, a smallish saloon doing the 1.2 Austin/2.0 Triumph range, a large one doing the 2.0 Rover/4.5 Jaguar range with a large sports car floorpan for 4.5 Jaguar sports cars. If the floorpans can be done easily in differing lengths then so much the better and if they can overlap with the van range then ideal. Clearly you can tweak the capacities up and down a little.
I see the biggest problem as being the politics of shutting down old sites and/or opening green field sites.