OK, there is no penalty (no form 5329).
Good. There should be no penalty because there is no early withdrawal penalty associated with a Roth conversion.
However the auto-generated form 8606 does not show conversion amounts on those lines. It shows the entire amount as taxable.
The entire amount is taxable, generally, so that's correct. But Roth conversion amounts can be withdrawn five tax years later without penalty, even before age 59.5. That's why they should be documented on the 8606 (particularly lines 16 and 18). It may be helpful for you later in certain circumstances.
Being that this is a conversion from an IRA started nearly 30 years ago where most of the gains are via compounding rather than contributions (there was a 10 year period with no contributions, for example), is it plausible that none of this amount would be taxable because it is all gains? If so how do I figure that basis out?
No. Unless you made non-deductible contributions (which is uncommon and hopefully you would know if you had), all distributions from a traditional IRA are treated as ordinary income. The age of the account, the contribution history, compounding, and gains are generally entirely irrelevant.
(In the uncommon case where you did make non-deductible contributions, you would/should have documented that on your tax return in Part I of Form 8606 and would track that contribution basis via the 8606 (and any related Form 5498s). Then any Roth conversions would be taxed on a pro-rata basis based on the ratio of your remaining basis and the value of your account. The ratio is calculated on line 10 of Form 8606.)