I'm thinking about a girdle where the 3 extra mains would be part of the girdle and then bolt this assembly to the block and be the main caps for the original 4. At the very least I will build billet main caps
With that billet crank then We can really turn the rpm level up.
I am having some M13 tool steel cam cores spun up to fix cam flex.
I have not got to put my hands on a block yet, off hand what is the bore spacing and cam journal diameter
The cam journal diameters are 1.8407"/1.8417" on all 4 journals. The housing bore for the cam bearings is 1.9715"/1.9735" for all four of them as well.
The bore spacing is not consistent from bore to bore because of the 4 main design causing a wider gap between cylinders #2 and #3, and cylinders #4 and #5. The spacing are approx. based on the measurements of some OEM head gaskets. 4.450" between #1 and #2, 4.520" between number #2 and #3, 4.450" between #3 and #4, 4.520" for #4 and #5 and 4.450" between #5 and #6.
Another fact you might not be aware of is the GMC block has main housing bore ID's that are (4) different sizes.
Also, a Comp Eliminator racer a few years ago built a 4.250" bore 6 cylinder. The stock bore spacing does not allow this, so he had to stretch the block length to accommodate that by using an LSM billet block. So you may be limited to a 4.125" bore.
