Hi,
Has anyone fitted factory front seats with integral seatbelts, say from a 3 series BMW convertible etc?
After advice on what anyone knows will fit.
Gonna need lots of extra strength adding to the pans of course.
Thanks

Depends what you are trying to achieve

Most of the modern seats now have pyrotechnics in them for seat belt restraints and whilst you don't have to connect them up it can make the seat base a bit too bulky to use in a Buggy. If going down this route though, be mindful of the shape and size of the seat belt tongue as not all of them latch properly if tugged hard. If using the belt as well from a modern car, again, be mindful of the exploding bits and watch your fingers as they are easily set off.

As long as you have a decent gauge floorpan I can confirm that is a very easy thing to strenghten each pan to fit seats/seat runners etc. Cut 100mm x100mm x 4mm thick steel plate and weld to underside of floorpans. (Must be continuous weld runs). A plate does not have to be "flat" it can be bent to right angle or any other angle for that matter or to a "Z" shape as long as the total lengths are 100mm each way (Up down angle left right.. whatever you need). As you can imagine when my buggy was scrutinised during iva examination this was one area that was looked at in depth. All was well and that part sailed thro`.....
(Note... the cheapo, skinny thin, floorpans are best avoided and will not pass inspection although I realise you are not interested in iva.  Skinny pans will not cause an issue with mot. All I am saying if it`s good enouh for iva it is strong and safe for you  ).