What's your asset allocation and what's your investment policy statement say first of all?
I have my whole portfolio at Fido, in about 6 different types of accounts and I treat them all as one big bucket, spreading my AA across them in what makes the most sense tax advantage wise.
If you have to put the entire 400k into a taxable account, then it definitely needs to be lean and mean as far as generating taxable events (less dividends/cap gains) and I believe Vanguard's ETF or mutual fund of the total stock market index fund (VTSAX) is going to be a slightly better holding than anything with FIDO and you technically can buy this at FIDO, but you will pay transaction fees since it's not a FIDO fund.
I hold the rough equivalent to VTSAX in my taxable account: Fidelity Total Market Index Fund (FSKAX). It isn't quite as tax efficient as VTSAX due to the proprietary patent that Vanguard holds (expiring in the next year or two I believe tho), but it is cheaper expense ratio, has no transaction fees/cost, and tracks the DOW more closely than VTSAX. It still is a wide range index fund but you'll see it move in a slightly different manner compared to VTSAX (I personally have no issues with it and very pleased).
There is another option for total stock/index at FIDO: Fidelity ZERO Total Market Index Fund (FZROX). This was introduced in the last few years and is a very good and attractive option, but word of warning: this is a proprietary fund. You absolutely can't buy this and then decide later on to move this fund/account containing it over to Vanguard if FIDO pisses you off for some reason. They will force you to sell all positions as they do not allow outside companies to buy/sell or hold their ZERO fund line. SO in the case of a taxable holding, ONLY consider this one if you are dead certain that you'll be happy with them for your investing lifetime, or be resigned to the fact that you may have a hellacious taxable event if you do need to move this down the road.
I've included a couple of links below that may help you figure out some ideas. But really, the main thing is to figure out your overall AA and then make sure that you rebalance the taxable stuff so it is streamlined/tax efficient, and distribute the non-efficient AA parts across the tax deferred accounts. This is another reason I LOVE having an AA/portfolio of only 3-4 funds. MUCH easier to figure it out when we're talking multiple accounts.
https://www.bogleheads.org/wiki/Fidelityhttps://www.bogleheads.org/wiki/Tax-efficient_fund_placement