Ask Reynolds for their tube list.
http://www.reynoldstechnology.biz/contact-us/united-kingdom/
Reynolds 525 cromoly would be OK for a head tube.
Part No. DC159L : 36.4mm O/D, 1.2mm wall, 600mm long
But Reynolds have a minimum order value.
or ask Ceeway for an equivalent.
http://www.framebuilding.com/Gara.htm
NVRM18
37, L=180, 1.6mm wall
600mm lengths available