I can confirm what Grumpy2 has said that it is a difficult job fitting MGF seats in a B. That is why people say they fit in a Midget but not a B
I removed the MGF mounting brackets from the bottom of the runners. Then I welded 20mm x 5mm steel strip across between the runners so that I could use the MGB mounting points. There was a lot of test fitting and measuring as they are a very tight fit because the inner sill of the B steps inwards in a way the Midget does not and the B inner sill curves in reducing floor width at the back and the tunnel curves in at the front.
I made the job more complicated by fitting seat heaters to the MGF seats while I was at it.
General MGB discussion