Mopar Performance book says a 340 measurements are:
Width
Outside valve cover to outside valve cover 20.5"
Oil pan sump 8.25"
Production manifolds 27.1" (some are narrower then this.)

Length
Bellhousing face to
Front of block (approx) 22"
Center of fan belt 27"
front of fan 31.8"

Height
Bottom of pan to
top of production air cleaner 29"
top of carb 26.5"
top of valve cover 22"
Pan rail to top of valve cover 14.5"

Approx weight 500lbs (about the same as a GM 350)
A 318 should be close to the same measurements.
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Just for the record, a 400: and a (440) : and a [225 slant 6] The book gave no numbers for a Hemi.
Width
Outside valve cover to outside valve cover 22.5" (23.8") [17.3"]
oil pan sump 8,25" (8.25") [-]
Production exhaust manifolds ,some are narrower 26.25" (28.8") [17.3"]

Length bellhousing face to
Front of block 23.4" (23.4") [27.7"]
Center of belt 27.2" (27.2") [28.0"]
Front of fan 32.8" (32.8") [32.3"]

Height bottom of pan to
Top of production air cleaner 29.7" (30.0") [29.6"]
Top of carb 25.5" (26.3") [26.5"]
Top of valve cover 22.2" (23.1) [23.8]
Pan rail to top of valve cover 17" (17.7") [19.7"]

Approx weight: slant 6 400lbs; 400 & 440 600lbs: 426 Hemi 700lbs and the older Hemi is heaver then a 426. Any aluminum added would reduce the weights.

Now, swap that Mopar into your hot rod. Gene