I think you are probably at the stage of making a new chassis as you've probably gone too far with the one you have to go back. Either fab a completely new design up, or find an original Minor chassis and use that. Advantage of a complete new design is that you aren't limited to engine size for the space available as you can fit whatever suspension etc that will do the job. Advantage of a new minor chassis is that it will fit the cab and all standard suspension parts can be utilised and be uprated. Engine would have to be a 4 cylinder, and the zetec option seems to be the current fave. Also with the current DVLA shenanigans, you may find registering a one off problematic, and it may be worth more still being essentially a minor if standard chassis used.
Your Mini plan would involve cutting the bulkhead and floor from the mini and the same from the minor and joining them. It's been done before quite a few times and can be successful, but takes a lot of measuring. No sure about using a mini though as I've never been impressed with them and they aren't very quick compared to an easier option. Have a look through the modified sections here and see how folk have built their cars as it may give you ideas.
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