I slightly trimmed a Buick white spring and it sits around 50-55psi. I could shim it with a washer and probably get it up to 60psi. The white spring in a Buick pump is around 70psi.
Unfortunately I don't know where to get just the spring (I looked at a few Buick parts sites). They come in the oil pump kits and the cheapest kits are around $20-$25 with free shipping. You may get away with yanking a spring out of any Buick 231/350/455 at the junkyard and shimming it a bit. They are 40lb springs, but way more substantial than the ones in the OHC. Since the OHC pumps are external, it'd be easy enough to try and if it isn't enough, it's not hard to change out.
http://www.ebay.com/itm/Engine-Oil-P...3D390972858130
Considering I trimmed the white spring, the green (stock) one isn't that far off visually. A few small washers in the cap to shim it.
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"How I learned to stop worrying and love the OHC Pontiac L6"
The Silver Buick- '77 Skylark coupe w/455, SPX, MegaSquirt 3 & TKO-600 (Drag Week 2011, 2012 & 2015!)
1969 Firebird with a turbo'd Pontiac L6 controlled by a MegaSquirt 3 and backed with a microsquirt controlled 4L60e and 4.56 gears! (Drag Week 2018!)