It never hurts to find out why someone has selected a particular design pressure/temperature, or grade of material. Sometimes it's been a pretty arbitrary decision. I've seen outfits spec grades of mild steel pipe that appear in tables in B31.1, but that nobody (around here, anyway) actually stocks. Meanwhile, there's another perfectly acceptible grade that's sitting in the suppliers yard or warehouse just down the street.

A105 flanges are very common, and readily available. I'd talk to the local inspection authority and verify what they take as the maximum ratings for 300# flanges. If they meet  your client's current requirements, there's no problem. If the 300s are just slightly under-rated, talk to your client and explain the potential cost & time of delivery differences. They won't be able to actually operate at the design conditions anyway, so they may very well drop the conditions slightly to make 300s allowable.

Some piping Codes like ASME B31.3 allow the use of standard flanges and fittings (from standards listed in a table in the Code) without further analysis.  It also allows standard flanges to be used at P/T conditions higher than the ASME B16.5 published ratings if the flange calculates OK per the ASME VIII-1, Appendix 2 method using B31.3 allowable stresses.  See paragraphs 303 and 304.5.  I have used this allowance several times to rerate existing flanges to higher P/T conditions that what the published B16.5 ratings would give.  This may be a way to justify using marginally inadequate (by the published ratings) Class 300 flanges instead of incuring the expense of the heavier Class 600 flanges.  

An alternate approach would be to modify your existing ANSI 300# Class flanges to incorporate a pressure energized bore seal ring such as Techlok type. These gaskets require less make-up force and retaining loads which increase the flange's performance and utlimate MAWP. These increases can equate to two or more pressure classes and are calculated in accordance to ASME VIII Div. 2 App. 4 and Div. 1 App. 3 requirements. The modifications are routinely undertaken by In-Situ machining contractors such as Furmanite and Team Industrial.

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