I am hoping someone could help with my unique problem.

My Son and I have rebuilt my late Fathers Ford 640 Tractor.   It is a 134 cu inch, 4 cylinder, 8 valve, gasoline engine manufactured in 1955.  We have completely rebuilt the tractor from front to back.  We replaced the rear main rope seal with a 2 piece neoprene Fel-Pro seal and had an instant leak.   The tractor forums suggest the Victor Reinz brand of seal but it was identical to the Fel-Pro brand and leaked just as bad.  After multiple tries and speaking with every tractor mechanic we could find we were no closer to finding a solution.  All of the older tractor mechanics in our area that once worked on these are no longer with us.

While surfing the internet I stumbled upon  Y Blocks Forever and the rare occasion of the seal grove in the block being deeper than the seal.  To verify this leak we have pressurized the engine block with compressed air and checked for leaks with a soap solution.  BINGO The rear main is leaking from the seal in the block.  I measured the seal grove and found this to be the case with this tractor and the cause of the leak.   The block groove is 2mm deeper than the cap groove allowing a leak between the block and seal. 

I spoke with Ted Eaton and Don Hyde on the phone and both recommended the 2 piece neoprene seal over the rope seals.  I have also found others that recall a shim kit that was available for this problem once upon a time but it is no longer available. 

Can anyone comment on the shims?  I am thinking I could manufacture a shim (metal, synthetic gasket material, or otherwise) to take up the additional space in the block and then seal the shim and rear main seal to the block and cap with Permatex Ultra Black Maximum Oil Resistant RTV Silicone Gasket Maker or Victor Reinz Aluminized Silicone Gasket Maker.  The Victor Reinz Aluminized Silicone Gasket Maker is manufactured to seal the rear main cap to the block instead of using crankshaft oil seal packing. 

Has anyone tried the shim method or have any other suggestions? 

Thanks in advance
Patrick Burchett