Unfortunately for white, 29. Rxb4 ..., is unplayable due to 29. ... Rc2, when black invades the 7th and pins the Knight.

After the exchanges and some maneuvering, 45. Nxa6 bxa6, leaves white with a permantently passive Rook.

Was 48. g4 ..., necessary? I suppose if the black King got to f3, it would be hopeless for white, so he adopted desperate measures.

I personally think along your lines; <48.g4> enabled Pachman to play <49.Kd4> since now there was no f5 pawn to protect that e4 pawn so Spassky was duty-bound to protect the pawn with his King and thus couldn't advance King to f3. (a lot of good it did for Pachman)

Perhaps he played it to also get his White Rook to kingside area but it wasn't going to be pretty for Pachman.

And finally, to answer your question let me first tell you how pathetic chess player I am and how, despite of that, I will give it a go since even Pachman couldn't save it.

Let's see... Perhaps I would go all-out:

<48.f4>

If Black now captures en passant, <48...exf3> (Black King can't approach the f4 pawn, delaying his intended march to f3), then I'll play <49.Rf2> to block Black f3 pawn. Now White e3 pawn is no longer blocked.

I guess Black's candidate moves are 49...Rxa5, 49...Ke5 or 49...Kg5

<50.Rxf3>

Targeting Black's f5 pawn and protecting White's e- and g-pawns. Now White King can play <51.Kd3> or <51.Kd4> due to Black not having pawn on e4 anymore, also depending on Black's 49th King move (if he played 49...Ke5 or 49...Kg5 or 49...Rxa5). Or...

<50.e4>

I like this move. Perhaps me being a sentimental fool! Anyways, <50.e4> attacks f5 pawn and I am hoping for <50...fxe4>, then Black King can no longer protect both kingside pawns if Black played 49...Kg5 (then White captures on e4 with his King).

<50.Re2>

Assume that after <48.f4 exf3 49.Rf2> Black played <49...Ke5>.

For example, say, <49...Ke5> and now <50.Re2> (Black can't play <50...Kxe4> because it's <<MATE>> and if <50...fxe4> then <51.Re3> and I am blocking new Black e4 pawn + defending g3 pawn.

I stop here since this is getting way too tedious (such a long post over a few moves!) I hope you look at this line <Rama>! Surely needs some polishing what I'm looking at. Just my try on getting White's Rook and King to kingside area. As you see, I didn't address queenside in any way so yeah, expect this to be flawed!