What is the cheapest way to make around 300whp?

[24OZ]

as i said,dedicated LPG will be the only way for high performance

and i would suggest using a couple of GRA gas carbies(and maybe twin B2 converters depending on HP goals) which are infinitely tuneable via their jetting system

if tuned properly there should be no problem with backfiring(that is common with the 'gas ring' dual fuel application)

i would also suggest a high performance spark system,MSD,ACCEL etc as LPG needs a good spark to ignite it as it has less BTU's and is denser and less volatile than atomised petrol

you can take advantage of more timing and higher compression with the gas only system