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gchuba- Mine is an 1987 SS converted to a custom CC, but otherwise a regular Montauk hull and liner.
After I removed my sump drain, I hole sawed from each side and saw that the liner was solid laminate plus gel coat, just like the hull side, but not as thick.
[If anyone would like, I'll post pics on my personal page]

Anyway, I wrapped the cloth around the tube ends to fill the over size hole [+/- 1/8" over], then overlapped cloth on mating surfaces max 1/2-3/4" beyond tube protrusion , making it a very rigid connection. My sense of the way most hulls are constructed, is that the 'transom-to-keel-to-waterline-to-sump/deck' is the most rigid area of the entire boat, so flexibility wasn't paramount to me.
Back when these boats were thought out, the brass tube idea was likely the least expensive, but every bit as rigid as a glassed in tube, except for those little o-rings.
Anyway, it took me about 4-5 hours of cutting, measuring, laminating, fairing, sanding, re-fairing, re-sanding, finish coating- but I'm done for the life of the boat.
My guess is that fitting a brass tube may have taken 1-2 hours.
Fast forward to lately and BW switched up to PVC, but not sure.
I wouldn't do PVC, even with epoxy resin, b/c I don't think the chem is compatible, eventho I haven't seen much that WestSystem doesn't bond tenaciously to.
Just thought I'd share what works for me.

(fixed abbreviated year)