Thermostat housing bolts 2.25 petrol

Two things, starting to do some work on the engine of my 1979 series 3 and had the standard very annoying issue of one of the three bolts in the thermostat housing shearing In the head. Drilling it out slowly but wanted to tap out (although I suspect I’ll have to go bigger) does anyone know what size they are? Secondly, cross referencing the number on my block tells me I have a diesel block but it’s a 2.25 3 bearing petrol 7:1. How common was this? Does it mean it’s also possibly got the heavier duty diesel crank? I have been told it’s a recon engine. Thanks

1/4 unf
the blocks are the same so if its a recon just the block has been repurposed

Spend time geting the centre right, then start around 1/8 but be very carefull as breaking the drill really spoils your day. Your aim is to get up to the tapping drill size for 1/4 UNF (probably around 3/16) then pick the thread out like a coil, it can be done but it all depends on getting the drill on centre and square. Can you get one of those old drill stands and clamp that over it to hold it square? Alterntively make a jig to drill though.