Quote:

If I found a 87 era Dakotawith the V6 in it, will the k member take the V8 or will that have to be replaced.???

Rumblon

There is no replaceable K member on a Dakota. They are a one piece frame like the full sized trucks.
In a hot rod, the V8 in place of the V6 is not an issue, in the real truck, the 87 engine compartment was smaller the the 88 & newer and firewall and radiator clearance may become an issue.