i have a 72 chevelle that i aquired from the original owner[the little older lady] . never the less the car was well used, so to make to make a short story long i need to remove the poor worn out 307 and leaking t350 to gain access to the thick oil grease and dirt what kind of cleaners will be best to degrime this mess

I'd get a power washer after it first and knock the big chunks off. Maybe even get it steam cleaned. Blast it good before pulling that grime ball out to to keep yourself cleaner. Then it's more power washing, using putty knifes and scrappers, and finally simple green and brushes.

I echo that approach with the proposition that the first "blast" be done at the local self-serv car wash. Call them and arrange for some time in the bay or go at an off-time (we have one that stays open 'til midnight) and you can usually get it up on jackstands, wheels off, etc and really work around/under it. Better to let them deal with the grimy waste than muckin' up your own driveway or sideyard or whatever.

I would do it on my dirt driveway. It would help to keep the grass from growing on it.