Hi!
Not too sure of the difference in quality in the adjustable ones, which if you have a upright 1600, this is what you need as technically you need the removes the heads to fit the fixed tubes, hence adjustable ones available for this.
If you have the later flat engine, like the 1700, which is the same style as the T25, then pushrod tubes can be removed in situ, without taking the engine apart, just off with the rocker covers, rockers, pull out the pushrods, and then the tubes will come out.
There is a ridge or something which stops the tubes being removed on the earlier engines, so the adjustable ones are used if you dont wanna take the engine apart.
AFAIK anyway.
Cheers!
Alistair