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$3751 out the door but does not include intake, carb, etc...
I was also looking at the Engine Factory 383 which is around $6K, but is turnkey with intake manifold/carb etc. and dyno tested, but only carries a 1 year warranty...
Jasper is a good product, as a general purpose engine replacement. If you are trying to make horsepower, that's not the route many would take. But if you just want to exchange engines and go, it's not a bad option. Like you said, you would still have to change some of the parts from your engine over to it.
You could buy new internals for your engine block. You will have to take your block to a machine shop to have your block prepped. This option MAY be cheaper than buying a crate engine, or you buy the crate and improve on that.
They have been around for years. I would say they are OK for a basic "exchange" rebuilt engine for your daily driver but making performance engines is not their claim to fame.