I hope removing the LPG system wasn't too hasty... With a mixer system the 6mm line should have been adequate, very likely the solenoid adequate too (or cheap to swap to one that is), if lambdas a regular replacement due to an oil burning engine you could have fitted a manual flow valve in place of the stepper controlled flow valve and run the engine open loop, doing away with the need for lambdas.
Dunno if I helped in the past, very welcome if I did and I'm sure others would say the same.
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
2 miles A1, 8 miles M62, http://www.Lpgc.co.uk