What year 235 do you have?
The 1954 Shop Manual states: "Springs should be compressed to 1 1/2 inches at which height the KMO-607 spring tester should read from 155-165 lbs on 235 engines and 175-185 lbs on 261 engines. Weak springs affect power and economy and should be replaced if not within the above limits."
The 261 camshaft has higher lift than in the 235 and higher load valve springs are used. The 261 also had stiffer push rods, rocker arms, and rocker shaft than the 235. The 261 also had exhaust valve rotators (used on the 235 Loadmaster engines but not used on the 235 Thriftmaster).
My books do not have any info on pre-load spring height. Sorry. If no one posts this within a few days, I will ask a local engine shop and post it here.
My guess is that you cannot have too stiff a spring (if you shim the 45mm springs) but too weak a spring could cause problems ("valve float" at higher rpm's -> problems as described by racecarldad).
I just bought a set of 12 springs for a 235 for $28 (from Patrick's).
Good luck,
