300 D conversion problem, help needed !!

Hi guys (and girls)
So I've spent best parts of two weekends putting a good 300 d saloon (123 5 cylinder 617) and autobox into my 280 GE after spending £200. plus on fuel one week recently !
After sorting out a few problems engine runs fine, starts and drives and is Eager to go !(pulls like a train) BUT..............................no speed at all up hills or on a straight motorway with any small incline. I mean seriously slow up a hill.
Now, I know the engine was good, the saloon sped up hills and kicked down OK.I used to run a 300GD LWB a few years ago and yes it was slower than 280 GE's but nbot like this., am I missing something obvious ? axle ratios, wrong autobox or what ?
Any ideas ?
I love my Gees but too slow is no good (road rage inducing at 30mph up a hill !)
Cheers
Andy Jones.