I would talk to Darren at Damico about a De Shroud on the inlets about £275, then a T34.48 with group a YB 0611 headgasket, chip to suit either greys or siemens.
The T34.48 will do 400 all day long, but 450 would really be better with a T38.55 I would say.
The head mods really bring it alive low down, in town trundling along at 15/20 I am in 3/4 gear and VERY responsive accelration at this speed, I am running a T34.48, and get 150lbft@1500rpm 330lbft@2300rpm and it holds 330lbft past 6000rpm, it does NOT drop off at all, due to the very small amount of headwork.
Damico 01953 717784.
tabetha