The F1 has an extra 140mm high rail between the chassis and the crossmembers of the tray.
You should be able to unbolt the rail from the crossmembers and sit the tray directly onto the chassis rails. There is a piece of timber that sits between the crossmembers and the chassis rail.
You will need some U-bolts to hold it down. If you got the ones off the F1, they will be far too long.
I can take some pics of the differences between the F1 and the MK3 in the morning and post them up. Its a lot easier than trying to describe it all.
If by chance tou did get the F1 U-bolts and don't need them, I would be interested in buying them, as some of mine had very worn and burred threads. I was going to buy some mild steel rod to heat and bend, then cut threads and heat treat, but if I can get genuine ones, it would be a lot easier.
Pics to come.
Greg.