I'm having issue getting thick tin-lead 63/37 solder on Alloy 42 lead frame
Temperature effects the strength of the solder's adhesion to the lead frame, but not the final thickness of the solder deposited on the lead frame.
The final thickness is primarily determined by withdrawal rate.  In other words, the speed at which you remove the leads from the solder.  The slower you remove it, the thicker the layer of solder tinning the leads will be.  This is not specific to alloy 42 and 63/37 or anything, but is common to any solder dip process.
Also, increasing the solder adhesion will help.  63/37 has relatively weak adhesion to alloy 42 unfortunately, no matter what flux you use.  It will really only ever achieve about 50% the adhesion strength compared to 63/37 but to copper, for example.  If you have enough process freedom, use the highest temperature that you can, and leave the lead frame dipped for as long as possible.  Both temperature and time spent wetted by the solder at temperature increase 63/37's adhesion to alloy 42.
Plus this is the only chance to improve adhesion usually - during packaging.  When the component is actually being assembled to a board, temperature and reflow times are too low and too brief to improve adhesion except insignificantly, but the work around is to tin the lead frame ahead of time with solder, but at higher temperatures for a longer duration than is practical for electronics assembly.  That way, the stronger solder adhesion is already taken care of, and during assembly, one need only worry about solder's adhesion to the already deposited solder on the lead frame.  Which I assume is why you are solder dipping these lead frames in the first place.
If you still can't achieve the desired solder thickness by lowering your withdraw rate, you may need to consider an additional process step like a copper plate over the alloy 42 lead frame first.  This works well - any SOT package device that needs to dissipate a bit of heat has alloy 42 leads plated with copper to mitigate the poor thermal conductivity of alloy 42 for example.