With the 351C "Big-Port" 4V cylinder heads it is the modifications of the area below the seat and particularly the short-turn radius that will provide the greatest effect for the effort but forth; you can in all but the case of the greatest efforts pretty much ignore the rest of the port. Simply stated, all of your efforts would be to aid in the reduction of interference in the process of the atmosphere traversing through the passage, the inlet port runners though arguably are not the most efficient are of substantial area, and since the actual seating surfaces between the opening and closing flow control valving imparts the greatest restriction, the closer to this that efforts are extended the greater the gains.
Specifically, the O.E.M.'s 90-degree cut below the 45-degree seating angle (note that the original O.E.M. machining did not incorporate a 60-degree angle) creates too great of a change of direction within in the distance, this causing the laminar flow in close proximity to these surfaces to be lost, this first increasing the boundary layer and then inducing turbulence, thus an effective reduction in functional area, particularly at the higher velocity rates. Therefore generally it is accepted that in these instances, a larger valve head diameter will provide one the opportunity to correct this.
But then yes, in the case of the 351C 4V heads (those w/ 2.19" valves), they already present generous dimension, thus creating other possible concerns, so some conservatism is generally needed here as compared to that of for example the typical in-line valve head examples. Therefore, as stated by another, if your needing to purchase valves anyway, and if the job were in our shop, I would advise bumping the diameter up a bit; just that which would prove necessary to provide the area required for a better resultant effect from the valve job, particularly if bowl porting/blending is not to be incorporated in the effort, but this sum specifically would need to be established by the one executing the work on those castings.
And while on the topic of attempting to better the flow in the port, and with the thought of "transitions" and their possible negative effects, note that the greatest offender on that port tract is the 'short-turn' radius, which actually suffers from even qualifying as such, rather there is presented an absolute 'shear', which presents the worst possible effect on attempting to avoid inducing a turbulent result; so if porting is permitted, effort here will aid tremendously even if one doesn't do much of anything else in the way of porting.
Scott.
