I've got a set of Pro Parts headers that I had ceramic coated years ago but have never used. I've got a 498 RB going into a '69 Barracuda fastback, probably around 575 HP and eventually adding a 250 shot of NOS.

Are these worth cutting the needed fenderwell holes for better spark plug/starter access, better flow for performance, etc. or should I just get some TTI headers?

Haven't seen a set of those in a long time.
FWIW, the TTis in my 512" RB Duster (Indy EZ1 heads) were a very tight fit; steering arms crashed into them and they were also very close to the trans pan.
Plug changes 2-4-6-8 & trans adjustments were a PITA.
So I guess it depends on priorities and whether you're willing to cut the car.

It's up to you
I've used both and I wouldn't hesitate to cut the fenderwells unless the car was a M code 440 car

Depends on what heads you have, if they will fit decent. If you have a raised exhaust port, not sure they will fit. I have those headers on a 71 Demon, 440 with Indy EZ1 heads. they fit pretty good, although I had to ding #6 for plug wire access and changing the plug. I had them, so I used them. Other than that, they fit pretty good. Mine are 2"

I also had a 71 demon with the same combination 440 ez-1 heads with those headers and liked them. But my fenderwells were cut out too

Here’s some advice, especially if you’re going to be using nitrous. You’ll want the easiest access to removing, inspecting, and reinstalling your spark plugs. Pro Parts headers in an A-Body will be a PITA for allowing the aforementioned. I went through it myself with a 440 on juice 25 years ago with those headers in an A-Body. Put them up for sale, and combine that money with money you’d spend for TTI’s, and find yourself a quality shop that’ll build you custom headers that will fit your car perfectly without cutting it up, and also put the tubing in areas off the heads to allow you easy access to your plugs, etc. Trust me, you’ll thank yourself a thousand times for doing this.

i have pro parts on my 69 dart but use stock iron heads.plugs isnt too bad to get to.if angle heads was used a few plugs might be a problem.but i head can use smaller plugs.i like them for being under chassis.they can be tight around the torsion bar on drivers side

Thanks for all the input! Much appreciated!

My set has the 2-1/8" primaries.

I forgot to mention that I'm gonna be running Edelbrock heads with angled plugs.... from these pics (not mine) the plug access looks pretty good with Edelbrocks....

If you're going to be a serious nitrous racer then get rid of the inner fenders and use a strut front suspension mounted on the cage. That way you'll be able to work on the engine and the car will launch and go down the track.

Not a "serious" racer at all. Springs moved in, going with minitubs. Just putting subframe connectors and an 8-point roll bar in it.

Car is gonna have A/C, Gear Vendors overdrive and Sniper FI. Looking at 575-600 at the crank on the motor with a 200-ish dry shot. Basically a street toy.

If it is a street toy then use TTI headers. The smaller the better, maybe the 1 3/4 size.

I ran them for a couple of years on my 496" low deck in the Barracuda. They were ok, but I picked up a lot when I built my own headers. Of course that engine was set up for 7600 rpm shift points and was never a street car. If you are putting it on the street sell the pro parts headers and put in TTI's or better yet, for ease of install the try y types. I have a friend running these and loves them.

Try Y headers

-Dave

Ran them in two big block Darts street cars. 440 @ 550HP and a 505 @ 634HP. Best A body big block headers I ever used. easy to install also. The hole on the passenger side is small. On the drivers side cut it out carefully and save it so it can be welded back.