The difference in barrel diameter is about choosing projectiles. Spherical balls and bullets intended for the .45-70 are .457" diameter. Bullets intended for the .45 Colt and .45 ACP are .452" in diameter. At least, that is the foundation for these diameters. For best results, you may need to size your airgun projectiles to suit the particular barrel you are using.
Along with those two diameters are available bullet weights. The .457 bullets have options that are heavier than .452. The weight, or more exactly, the length are the primary driver for rifling twist rate required, for a give caliber. Here the .452 and .452 diameters are close enough that projectile weight and length are what matters.
Twist rates required are also affected by muzzle velocity. Faster twist rates are required right around the speed of sound. Shooting airguns around 1150 FPS is noisy and wastes air. So, 1050 FPS is a practical ceiling. For larger calibers, very few would shoot that fast, or choose projectiles light enough to shoot that fast. You should choose the projectiles that group the best, rather than insisting on a given velocity. That might be anything from 700 to 950 FPS with larger bores.
To get to the point, 1:24 twist should be good for up to 400 grain bullets, if they are blunt, as used for handguns, or the .45-70. A 1:32 twist might limit your bullet choice to 250 grainer and lighter, depending on actual shape and length. The limit being stable flight and tight groups, with a clearly under spun projectile leaving long holes in paper due to extreme yawing, or tumbling. Over spinning bullets does not seems to cause trouble; although best accuracy is achieved by not doing that - assuming the rest of the system is good enough to show a difference. Under spinning bullets blows groups, so is more of a potential problem.
One could argue that the bullet weights I gave above are 50 or 100 grains too light. Rather taking this as law, consider it in principle. If you want to shoot pellets primarily, slower twist is better.
I think you are correct about being able to adapt a .457 caliber PCP to a .452 barrel, if all of the interfaces are correctly machined. Your bullet choices and twist rates should be compatible.
Now, while theoretically possible, airgunners tend to shoot soft lead cast or swaged bullets, rather than "hard cast" ones made for firearms. So, the bullets may be cast from the same molds, but the alloys used for airguns are softer. This is to reduce the engraving force on loading and more especially as the bullet starts to accelerate. Else, the air pressure may not be high enough to get the bullet moving. There are tricks to make such jams less likely, such as a long taper in the chamber, with the bullet pushed in just deep enough to hold it in place. Then, when air pressure is applied, the initial engraving force is low enough for the projectile to gain some speed to punch its way into the full rifling depth.
In an apparent contradiction, the bullet can be loaded deep enough that the rifling is fully engraved, thus the loading probe has done most of the hard work, so air pressure will be able to get the bullet moving. A lot of force applied to the loading prove risks distorting the bullet and making it group poorly.
The Texan is loaded via finger force to the bullet only, so the projectile needs to be relatively loose in the breech.
The above rambling is not intended to cover all factors. Just to indicate that there are many consideration that need to be made, before a given barrel change can be "blessed".
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