They're NOT classical forms. We often use them.
And the transforming is not good. It's not wrong, but, as you can't find anything about them on the internet, there's no law in these instances. You should learn about conjugation of verb, adjective and NA-adjective, and you can take sentences to pieces of words.
よかろう → よかろ う
神であろう → 神 で あろ う
言えよう → 言え よう
"う" and "よう" always are after "the imperfect form" of declinable words and add a meaning of guess or intention to the words, and "だろう" is a shorten version of "であろう," so all of "だろう" can be transformed into "であろう."
"で" is used after noun or "attributive form" and there's no distinct difference between using "the imperfect form + う/よう" and using "noun/attributive form + で" when you use it as intension, but just nuance.
"Noun/attributive form + で" cannot be guess.
"VERB + う/よう" can be intention but adjective and NA-adjective CAN'T.
よかろう ≈ よいだろう = よいであろう(sounds a bit odd)
神であろう = 神だろう
言えよう ≈ 言えるだろう = 言えるであろう
書くだろう = 書くであろう ≈ 書こう
食べるだろう = 食べるであろう ≈ 食べよう
するだろう = するであろう ≈ しよう
来るだろう = 来るであろう ≈ 来よう (Verb, but only for guess. Look up "行く" and "来る." Their ranges of meaning are different from English ones.)
安いだろう = 安いであろう ≈ 安かろう
綺麗だろう = 綺麗であろう ≈ 綺麗だろう (As there're many theories of NA-adjective, you can say either the left one and the right one is the same or not.)
車だろう = 車であろう