Number of seats up to 7 is not an issue.
All seats have to be "proper" seats firmly fixed to the vehicle.
Things get ify if you exceed 8 seats, then you become a minibus & new drivers have to pass a minibus test.
Folks with 10 & 12 seat Land Rovers can have problems, but like the driving test, have "grandfather" rights.
... and don't think you can get away with removing the seats
Some of our Station Wagon owners will have better info.
You have to define "modified" and the context you use the word to describe the vehicle in question and to which agency.
If we are referring to the MoT exemption and retaining Vehicle of Historic Interest to qualify for tax exemption then as far as engine conversions - conversions that were common 40 years ago are acceptable. So that's Rover V8s, Essex 6 pot, Transit & BMC diesels, Perkins
diesels, including the Perkins Prima, 200 & 300 Tdi's etc.
You could fit another engine type, but would lose MoT exemption.
Modified as per the DVLA points system, means you could/would lose your VHI status and have to pay Road Tax (VED).
However, if you have documentary proof the changes were made 40 years ago, you might be OK. The DVLA would want the vehicle inspected by their agents to ascertain what it is.
Building a "one off" special makes you liable for an IVA test and compliance, and probably a Q-plate.
For the absolute low down you need to go to a forum that specialises in kit cars and the like, extreme modifications are a bit outside of our experience.
This topic being about insurance; Insurers need to be told about any modifications to the original spec of the model. The "common" engine conversions of the day don't normally attract an extra premium - V8s and that. Fitting winches, servo brakes, or even the S3 brakes, hard top, truck cab, tilt. They just need to be told so they have the details on record.