In the 1990s there were no issues with authorities swapping engines around to keep an old car going. However, nowadays government relying on people spending their money onnew cars the have become rather unwilling to let you keep your old banger and keep it going as long as it can theoretically go for.
However having said that it is still possible as long as modificaitons to fit the engine is kept to a minimal and with some effort put into the designing of mounting points to ensure it will not give up unexpectedly.
For that purpose I would make everything at least 3 times as strong as it needs to be bearing in mind that steel is 2 times as strong as Aluminium but 3 times heavier. so for that if there is a section of Aluminium 10mm thick instead of making up a steel bracket 5mm thick make it with 8mm steel.
This is to compensate for the lack of stress analysis which are normally carried out buy automotive manufacturers.
Basically what needs to be done is to call a vehicle insurance assessor who are experienced in assessing special vehicles such as cars with certain modifications such as hearse conversions, ambulance conversions etc to have a look at the vehicle if it is insurable.
The followings are the kind of things that would make assessors ask difficult quesitons.
a) Messy weld joints
b) Unsecured or unfused electrical wiring
c) Sharp objects on the exposed surface on the interior and exterior
d) Unsecured fuel pipes
e) Exposed interior wiring
f) Obviously inadequate electrical rating for starter and charging wiring
g) Obviously inadequate heat protection
h) excessive alteration to chassis
i) Obviously inadequate cooling capability
j) Obviously altered suspension and axle components
Once those items are deemed safe the rest is easy.
Once all aspects are covered the assessor will issue a single page report and pay the fee and the car will be insurable.
The reminder of theprocess is the same as putting a car through an MOT/NCT test.
By the way, don't forget to bring the letter to the test centre with you.