You spark problem is in the crank shaft sensor or flywheel sensor, And this is were you need to find them on the driverside of the transmission right before the slave cylinder there are 2 lines replace them the car will run. now about fixing your car, I have a 1984 633 that I drive everyday and a 1985 635 that I am restoring my 635 was hit in the front you can only get the front end off of a 6 not a 7, take the car to a bodyshop and let them drill out the spot welds and put a new one on and fix any other problems that you may have. DONT CUT THE RAILS OFF and weld in a new front your car will now be shit and if you ever want to sell you 6 no one will want it DO IT RIGHT THE FIRST TIME, It may be more money but its the right thing to do. You should be able to find a front end in a yard but new you are looking at $1.150.00 or $1.500.00 and from what i found out there are a few in the USA and you would be looking at 2 weeks to get one if not 3-4 weeks from Germany that if you go and see you local BMW dealer. Now if you have the time and money and you want to change your auto to a 5 speed, look at a 5 speed from a 6 and see what you are going to need I know what you are going to need and it big money my friend BIG MONEY and lots of time. for a rebuild kit you want to call Metric Mehanie or web page look and see what you want to do and I think I know what you want its BIG MONEY. Now when you say custom do what you like but here are somethings to remember the 6 all ready needs a 300 pound dieit, is this going to be a street car or a track car,......look man we have to talk you need my help email me at
collinmckeller@hotmail.com give me your phone number so I can call you, Because I may say something that may upset others. But I want to help you and save you some money