The Q plate won't happen JUST because of the need for an IVA.

If you have a standard car and have modified the chassis to fit a 5 speed bx, but have the same engine, steering, axles etc you should keep your own reg.
If you have used a shell and bits from one or more cars you will get a Q, not an age-related plate.
If you used a new shell and most bits from a single donor you would get an age-related plate. Multiple donors would give a Q.