"Should probably be" vs. "should be probably"

Which one is more correct?

This one should probably be similar to the other one.

This one should be probably similar to the other one.

My gut tells me it's the first one, but I'm not sure.

Compare it to other example, like:

I should probably do this.

I've never heard "I should do probably this."

According to this, the first one is correct:

This one should probably be similar to the other one.

Both phrases look equally unwieldy to me but a Google search yields about 25,200 results for "should probably be similar" vs. ONLY 248 results for "should be probably similar". That's about three orders of magnitude apart from each other.

"Should be" is the predicate, and "probably" is an adverb; you can write "probably should be", or "should probably be", but because modifiers in English generally come before the word modified, it is ungrammatical to place the adverb after the verb.

There is no trouble in inserting "probably" between the "should" and "be"; "be", here, is a modal verb complement to "should", not an infinitive. There is nothing ungrammatical in the least about splitting modal phrases with modifiers. "He would really liked to have been...." "He would have really been better off...." "He really would not have eaten...." etc. All of these are perfectly grammatical.

It is possible, however, to manipulate the sentence with some grammar, and give it a colloquial, spoken scansion:

"It should be, probably, ten feet long."

Which would be the same as saying something like this:

"It should be, oh, probably ten feet long."

In spoken English, one might well hear someone speak this way, and the speaker might well speak so quickly as to omit the pauses represented by the commas; in written English, for the above-mentioned reasons, though, you can't really do it. You'll need the commas to indicate the syntactical breaks.

Basically, "probably" in the latter instances is being used as a synonym for the adjective "about", and so is functioning not as an adverb, but as an adjective; in the former instances, it is being used as a proper adverb. That's why you need the commas to distinguish the latter usage from the former, more formal usage: they separate the word "probably" from the predicate phrase, so the reader can associate it with its intended object, the following noun phrase.

Adverbs are often adapted to adjectival use, so this isn't really all that strange a situation.