Manton makes single tapered pushrods
On the stock rocker arms make sure and check every one of them for clearances, those stock rocker ratio vary a bunch
I check around 4 different sets of stock rocker arms back when I raced a street hemi in NHRA stock, I check all the rockers on the same cam using the same pushrods on one side of the motor NHRA allowed .484 lift at the intake valves and I think .464 on the exhaust, my finding where the exhaust varied from .430 to .530 and the intakes where as bad as the exhaust rockers where at the valves, the ratio where no where near each other
I've seen similar findings on other after market BB race rocker arms also, expensive ones too