I have used the Gliptone colour for many years in different cars without any problem, even in the extreme heat we had this summer here in Sweden.
But if the leather isn't really clean before the colouring it may be sticky according to the manual.
If you use the Gliptone conditioner it might seal...
There is no problem in using a short neck water pump instead of a long one.
It's only the fan blade that will be I bit far behind the radiator, the pulleys will align.
I have used a short one on my 2200 TC 1976 for many years and no heating problem.