I've learned the hard way - oil pump gasket THICKNESS is critical. I learned from a retired dealer mechanic that the GM gasket is .009" thick. I've mic'ed gaskets included in full sets - THEY ARE THICKER, around .014"!!! Of course, I learned this after spinning a rod bearing on a 3/4-race 235, with GOOD clearances - .002-.0025" across for rods and mains, but never has decent pressure at road speeds - even after replacing the pump with a near-new part in the car. That job was a PITA!!!
My current 235 in my '57 car has a "sloppy bottom" (rods at .003", mains at .004"+), but did have a GM oil pump. I made my own gasket from a file folder that mic'ed at .008". Idle pressure is on the low side like your '59 engine, but perks up to 28-30lbs HOT. Oil is Castrol 20w-50, with a bottle of STP to keep the cam and lifters alive.
Tom Langdon seems to prefer the GM pumps, and still might have the GM gaskets. I would check the gear to housing clearance (use V-8 specs, or contact Tom for advice) and stay with the GM pump. I used sealer on both sides of the gasket, then removed the cover to clean up the squeeze-out and back together. So far, SO GOOD!!!
I also have a 5-window, a '53 I bought in 1974 at 16, and my next build will be a 261 I bought in a basket that looks really good for wear and still at STD. Ha!!! Keep us updated with your build progress!
Good Luck, and MERRY CHRISTMAS! Tim
