The 3 litre 603 lower block is the way to go for many reasons. Also once installed it will increase the value of the cars still having the 3.5 in my opinion. At least to potential purchaers that know these cars in the future.

Costwise it is probably as economical as any other option or more so. Nothing is bulletproof but with your later head it probably becomes as close as your going to get.

The 3.5 engine should never have been produced as you never know if it is going to hold up or not. Even after a rebuild. You know the 3 litre lower block will with rare exceptions.

I too am retired and needing more frequent servicing to stay above ground. Repair of a bulging abdominal artery is the flavor of this month. Just waiting to go in.