You will have a brake imbalance, with the large callipers on the rear you will have a tendency to lock up the rears, not too much of a problem if perfectly straight when braking…….disastrous if you have any steering angle applied.
The advantage of sleeving over replacement is that the cups are infinitely rebuildable. However, if you maintain your system by changing your brake fluid every few years, it is pretty likely you will not have to worry about that. I personally prefer to sleeve - Apple Hydraulics has done good service for me (no affiliation), but there are others. It doesn’t matter how bad they are - the sleeve takes car of the pitting.
As to moving the larger to the rear, I haven’t done it, but think it might be a good idea if you are upgrading the front calipers. Others have done this. I have 420 Girling front calipers on my Mk2 and like them very much, but they are hard to find, and require the uprights to be able to bolt on. If you do find a parts car for these consider also changing the pedal box to run the later 4.2 master cylinder, same as E-type.
I recommend leaving the brake setup alone, there’s only pain and trouble in going away from the original engineering unless you’re really knowing what you’re doing. You don’t put stronger brakes in the rear without correcting the brake balance or you risk spinning out and losing control and performance.
My brakes work great and I bleed them on my own with a tube and a jar.
Front brakes must almost be more powerful than the rears as discussed, and I also recommending:
New calipers
New brake lines not a bad idea
New everything ideally!
Look into gravity bleeding
No oils anywhere, only ever use brake fluid once the parts are clean and dry
And most important of all, anything you don’t feel absolutely confident in, don’t do it. Either get help or learn it before you need to rely on your first attempt.
Appreciate every comments…and David…you are right…
I go for new calipers and new brake master because it was it clog dirty and seal unservible …well 20 years stoped it’s a lot … thanks for yours patience about with me …
Pd…sorry if I ask questions that may seem silly…
but it’s a first project I’m doing … and I’m doing it because I like mechanics … but hey … here I am …
I saw a sign once,’the only silly question is the one that wasn’t asked……it may save your life!’