im pretty sure a windsor box is different to a clevo , what are the differences and can a windsor box be modified to suit clevo

what flex plate convertor bellhousing etc

SAME...

Some very early C4 Windsor autos have 5 bolt pattern and some Windsor C4's use a smaller bellhousing/convertor, probably still fit but.

Don't use a Windsor flex plate with a Cleveland, they have a different counter balance weight.

Yes it's true early windsor C4's used a smaller flexplate(157T) and a case fill dipstick tube.
Later C4's as used on clevelands and 351W's have a 164T flexplate and correspondingly larger bellhousing which has it's own separate bolt pattern whereas the early small bell, pan fill C4 has it's bell housing bolted on by the pump bolt circle.
Early 289/302/351w and Cleveland engines all have a 28oz inbalance. Only late 5.0 windsors(from about '85 from memory) have the lighter crankshaft(smaller centre counterweights) and therefore require the 50oz imbalance.

Pete.