T4 'head flow characteristics/valve sizes

[MattKab]

I have two pairs of T4 'heads. A pair of 021 101 371 S with 41*33mm valves. Also a pair of 071 101 371 A with valve sizes 40*33mm.

I may consider resizing the valves to suit my plans for a 2056 revving to little over 6krpm using a single TB and plenum behind a 911 fan.

Am I right in thinking that the exhaust ports on 021 heads are less restrictive than those on the 071, which look deeper?

What would be good valve sizes for my application?