Matching new carb holders to carbs and head.

Howdy all. Recently bought some new carb holders for my built 77-kz 1000A, it's running 33 vm smooth bores that have been bored to 38 mms I believe. That leaves a bit of a wall where carb seats against boot. Will machining the interior of the boots to match the carb weaken the boots. They will also have to be opened to match the head removing more material. Are there any other holder options that would work without machining ?  Forgive my ignorance and thanks ahead. 

The rubber manifold is bonded to a steel flange and there's plenty of thickness to play with.
You can use a high speed sanding roll/flapper disc etc to open the rubber to match the carb bore and then blend it to the port.
I do it all the time to those J manifolds with no issues.

Thank you Zed. I guess the head would have to removed to do that properly. 

Don't see why.
You are only removing material from the manifold and not the head.
Just open the manifold to match the carb bore and blend it so it tapers to the boots existing outlet diameter.
This can be done with the boot off the bike.
A slight step up from a manifold diameter to a larger inlet diameter is ok if you can't get it to match the inlet port.
What you must avoid is a step down from the manifold diameter to the port diameter which will disrupt gas flow.