Most people lower tractors for more stability - its best for faster applications. My mod murray was stock height and was seriously scarey to drive at any sort of high speed - as a cheep lowering mod on my one MTD, basically i raised the front axle to the top of the chassis, bolted it solid ( there wont be any flex - you dont want any at high speed) , for the rear i just used some low profile rear tires.
I also bolted the seat to the chassis- the thing handled like a slot car- was so much better handling- cornered nice n clean.
My next couple mods will be lowered using the same solid front axle, custom building front lowered spindles, using either a 700 peerless or right angle gear box to a straight rear axle relocated with steel tubing , pillow block berings for about a 4" ride hieght ( with wheels on).
Probably best way to build a lowered tractor with 'flex' or adjustable suspension would be to do like rockcrawlers do - linked suspension with hiem joints and a locator bar ( to keep axles straight with the body) - steering setup would need all hiem joints, would be best to use either a peerless 700 or RAGB to a straight rear axle , but belt driven ( chain would twist) or a shaft drive trans.